Fish Assassin
Crew Member
Sure wasn't meant to be........ Got more props to test
But I have wondered since day one if my motor was mounted at the right height
Prop Advice
- Thread starter Fish Assassin
- Start date
But I have wondered since day one if my motor was mounted at the right height
salmonkiller01
Well-Known Member
I was lead to believe the cavitation plate should be 1/2 inch above the bottom of the boat when an outboard is mounted to the transom.
Fish Assassin
Crew Member
And mines level.......... If think part of dialing this in, is it needs to go up a hair
Just hate to go up and then start all over with the props
wolf
Well-Known Member
Trim tabs will help and you will love them as it really will help unbalanced loads on your boat ,I put some on another forum members boat and questioned himself why he never did it before and its a lil 19 fter.
Dont know how you rip them off?? never has happened to me but id be more worried about my boat and leg if they are getting ripped off IMO.
As for props I really experimented with one of my boats before make sure you are in the middle of what WOT rpms are you dont want to over rev your engine or under rev it.
Whats your range anyways? where it says on the motor specs?
Dont know how you rip them off?? never has happened to me but id be more worried about my boat and leg if they are getting ripped off IMO.
As for props I really experimented with one of my boats before make sure you are in the middle of what WOT rpms are you dont want to over rev your engine or under rev it.
Whats your range anyways? where it says on the motor specs?
Fish Assassin
Crew Member
Max is 6000rmp
So I've been testing hole shot, 4000rpm, 4500rpm and 6000rpm
Best so far is the 4 blade 14.5X17 all problems solved without trimtabs, but I lost speed almost 5mph at all ranges
Going to test ported over non ported in 3 blade next then SS over aluminum after that.
My cavitation plate is right inline with the bottom of boat. Thinking bringing it up .5-1" wouldn't hurt and it should remove some torque steer or so I have been reading.
Many hours googling and testing....... Yes I am just being anal, but why not. There's ok/good which I alreday have and there's freaking awesome. I am aiming for freaking awesome. I could just except what I was given or test things till I nail it.
Thanks for all your help,
So I've been testing hole shot, 4000rpm, 4500rpm and 6000rpm
Best so far is the 4 blade 14.5X17 all problems solved without trimtabs, but I lost speed almost 5mph at all ranges
Going to test ported over non ported in 3 blade next then SS over aluminum after that.
My cavitation plate is right inline with the bottom of boat. Thinking bringing it up .5-1" wouldn't hurt and it should remove some torque steer or so I have been reading.
Many hours googling and testing....... Yes I am just being anal, but why not. There's ok/good which I alreday have and there's freaking awesome. I am aiming for freaking awesome. I could just except what I was given or test things till I nail it.
Thanks for all your help,
salmonkiller01
Well-Known Member
Before you raise it ask the shop your dealing with the props if raising motor height will cause porpoising
nicnat
Well-Known Member
my boat is similar size and hp, my plate is almost 2" higher than the bottom of the hull but I have a fairly deep vee, my outboard manual states "conventional v-hulls often perform well with the anti ventilation plate approx. 1" above the bottom of the hull" i've had a few different props on and have had no porpoising issues at all, not podded like yours though. also check with your merc dealer for the exact optimum rpm range for your motor at wot, it may be a little different than the specs. my specs state 5500-6000 but the mechanic looked up the optimum for it for and came up with 5450-5750 at wot. my aim for the right prop is to be in that range and have the best mileage at my main cruising speed for getting to the fishing grounds and back. i'm not concerned really with my speed at wot as i will rarely use it . you want to be able to hit that sweet spot for fuel economy, speed and rpm, for mine it's somewhere between 3500-4000 rpm in the 25-30 mph range, not quite dialed right in yet as i don't have a flow meter.
Sharphooks
Well-Known Member
Everything you say is correct nicnat, but not sure it's just a question of WOT. I ran a 14.8 x 19" for several years on a Honda BF150--- under perfect conditions I could get the WOT of "upper 5's" usually associated with the bigger 4 strokes but I started thinking about what the normal RPM's SHOULD be for the outboard during NORMAL cruising conditions.
With that 19" the comfortable cruising RPM's was 3,700 but the more I researched what the nominal operating RPM's should be, I realized that I wasn't doing the outboard any favors by spinning such a large wheel at relatively low RPM
So this past November I stepped up for a brand new BF150 (with the NMEA output so I can now track RPM and fuel burn rate on my MFD). I also got rid of the 3 blade 19" and went for a 15" x 17" four blade. Now, the "comfortable" cruise RPM's (at the same vessel speed as before) is 4,000 and I have no problem pegging the limiter at 6,200 at WOT
Yes, my fuel rate burn has gone up but in the long run, I think the outboard will thank me for it-~~I continually hear from mechanics that 4 strokes like to be ridden hard at the upper ranges of their power curve
Owen, you have an impressive cash war chest to have all those props at your disposal (or a very generous prop shop guy)-~~it didn't surprise me to hear dropping down your pitch a bit and going to a 4 blade helped address your issues. But I think the advice you're getting is good advice~~ trim tabs, or raise the motor a notch, or both. I'm not convinced a prop pitch or rake or diameter change will completely cure the porpoising you described
With that 19" the comfortable cruising RPM's was 3,700 but the more I researched what the nominal operating RPM's should be, I realized that I wasn't doing the outboard any favors by spinning such a large wheel at relatively low RPM
So this past November I stepped up for a brand new BF150 (with the NMEA output so I can now track RPM and fuel burn rate on my MFD). I also got rid of the 3 blade 19" and went for a 15" x 17" four blade. Now, the "comfortable" cruise RPM's (at the same vessel speed as before) is 4,000 and I have no problem pegging the limiter at 6,200 at WOT
Yes, my fuel rate burn has gone up but in the long run, I think the outboard will thank me for it-~~I continually hear from mechanics that 4 strokes like to be ridden hard at the upper ranges of their power curve
Owen, you have an impressive cash war chest to have all those props at your disposal (or a very generous prop shop guy)-~~it didn't surprise me to hear dropping down your pitch a bit and going to a 4 blade helped address your issues. But I think the advice you're getting is good advice~~ trim tabs, or raise the motor a notch, or both. I'm not convinced a prop pitch or rake or diameter change will completely cure the porpoising you described
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SpringVelocity
Crew Member
Roy...your putting them on mine so you know....
Listen to Wolf he has fixed a lot of boats with same symptoms Owen if you don't want to put in dole fins drill into that new motor than trim tabs are only way.... I can't see this prop thing saving you..Or shift some weight around.... I have sort of same symptom as yours except I have a kicker mounted on my starboard which was bad choice..So when I by myself the boat will list.... Only way to fix is move kicker which I don't want to do or put on ttrim tabs...
Fish Assassin
Crew Member
It will happen without trimtabs. When I nail it I'll share the result and say I told ya I didn't need them LOL
No trim tabs will be installed.
Problem was solved yesterday without either tabs or hydrofoil. I just lost 5mph..... I am really close
Its either prop or motor height. If it's prop that's best, if I need to try lifting the motor .5-1" so be it
LOLNo trim tabs will be installed.
Problem was solved yesterday without either tabs or hydrofoil. I just lost 5mph..... I am really close
Its either prop or motor height. If it's prop that's best, if I need to try lifting the motor .5-1" so be it
Sr SQ
Well-Known Member
Correct motor height can only be guessed at until you take it for a run, you can get close with general rules but every hull will differ slightly and will need its own real life assessment.
To assess it you need to run it at planning speed with average load and look at the ventilation plate, it needs to be riding on the surface of the water with minimal spray from the leading edge.
If its not, or you have so much spray that makes it hard to see than your motor is to low.
You cannot properly cure motor height issues with propeller changes, only mask the height problem.
I seen speed increases of 5-7mph at cruise just from correcting motor height on a factory installed motor.
To assess it you need to run it at planning speed with average load and look at the ventilation plate, it needs to be riding on the surface of the water with minimal spray from the leading edge.
If its not, or you have so much spray that makes it hard to see than your motor is to low.
You cannot properly cure motor height issues with propeller changes, only mask the height problem.
I seen speed increases of 5-7mph at cruise just from correcting motor height on a factory installed motor.
wolf
Well-Known Member
Well you should have a "range" by dealer specs for that engine for instance my old outboard it was supposed to be between 4500 to 5000 Rpms so I matched a prop that went 4800 WOT with weight on boat a little higher as you will have gear,people,gas, etc so you want to be in the "mid" range as you do not want to lug your engine or over rev it that will cause problems down the road,
Trim tabs help in other ways too especially in uneven weights on boats FYI
Good luck Wolf
Trim tabs help in other ways too especially in uneven weights on boats FYI
Good luck Wolf
tykkyt e blue
Well-Known Member
I agree with Sr SQ ( "You cannot properly cure motor height issues with propeller changes, only mask the height problem" ) - good improvements are available from best motor mounting heights .
Many years ago, I gave up on the belief that the cav plate needs to be even or 'near even' to the hull bottom. The critical issues are keeping a smooth solid flow of water past the main majority of the prop blades surface ( small amount of slip is present at all times ) and an acceptable supply of water to the cooling inlets. every boat is different enough that testing and observation is the only answer.
You can raise the motor LOTS in some installations. You can have good 'holding' of the blades and still have good water pressure to the cooling sys ( water psi gauge is your friend here ) get better speed, better economy and have no ride complaints.
Unless the prop starts to slip in sharp turns, getting on plane, lose the ability to trim out at speed, or lose cooling water pressure - raise the motor.
Both of my boats run best 2 1/2 - 3" higher than the manf. recommended installation heights. The alum runabout has a hyd transom plate so I have done lots and lots of experimenting with motor heights. The only reason the motor on the Monaro is not higher is because its at the top holes and moving the transom bracket will make 'swiss cheese' of the transom drilling all the new bolt holes involved.
Something that is very hard to observe is how much the water 'rises' as it breaks from the trailing edge of the transom. The hull 'pushed' the water down to provide the lift to keep on plane. As soon as the water escapes from under the hull, it rises back to sea level. How much it rises before reaching the propeller varies with each boat configuration and speed. Set back distance ( length from transom to leg ) also has an effect - more length means more time for the water to return to level.
So setting the motor height relates to how the water exits from under the hull.
I don't know if this will help find the best set up for your boat - but the fact that you lose so much performance in 'curing' the porpoising suggests a set up issue not prop issue. The loss of speed is excess prop slippage for that propeller in that set up.
Many years ago, I gave up on the belief that the cav plate needs to be even or 'near even' to the hull bottom. The critical issues are keeping a smooth solid flow of water past the main majority of the prop blades surface ( small amount of slip is present at all times ) and an acceptable supply of water to the cooling inlets. every boat is different enough that testing and observation is the only answer.
You can raise the motor LOTS in some installations. You can have good 'holding' of the blades and still have good water pressure to the cooling sys ( water psi gauge is your friend here ) get better speed, better economy and have no ride complaints.
Unless the prop starts to slip in sharp turns, getting on plane, lose the ability to trim out at speed, or lose cooling water pressure - raise the motor.
Both of my boats run best 2 1/2 - 3" higher than the manf. recommended installation heights. The alum runabout has a hyd transom plate so I have done lots and lots of experimenting with motor heights. The only reason the motor on the Monaro is not higher is because its at the top holes and moving the transom bracket will make 'swiss cheese' of the transom drilling all the new bolt holes involved.
Something that is very hard to observe is how much the water 'rises' as it breaks from the trailing edge of the transom. The hull 'pushed' the water down to provide the lift to keep on plane. As soon as the water escapes from under the hull, it rises back to sea level. How much it rises before reaching the propeller varies with each boat configuration and speed. Set back distance ( length from transom to leg ) also has an effect - more length means more time for the water to return to level.
So setting the motor height relates to how the water exits from under the hull.
I don't know if this will help find the best set up for your boat - but the fact that you lose so much performance in 'curing' the porpoising suggests a set up issue not prop issue. The loss of speed is excess prop slippage for that propeller in that set up.
Fish Assassin
Crew Member
I moved onto and completed diesel heater install. Now when testing props I'll be toasty warm
Was going to do tests today but the wind was blowing right up the puntledge figured I'd leave it on the trailer. Might try tomorrow afternoon. I just won't do on weekends as ramps are to busy. It's hard though as when the weather is good I want the fish and prawn not test props
Was going to do tests today but the wind was blowing right up the puntledge figured I'd leave it on the trailer. Might try tomorrow afternoon. I just won't do on weekends as ramps are to busy. It's hard though as when the weather is good I want the fish and prawn not test props
Fish Assassin
Crew Member
Well I have not found a prop that beats my 14.5X19 Merc. 3 blade yet that came with my motor..... I have now tried 5 other props and will be speaking with an expert early next week......
Just finished testing 3 other props.
Just finished testing 3 other props.
Fish Assassin
Crew Member
Test was done with a pretty full load......
4 blade props pop the rear up quickly but then went no where...... 6000rpm in 6 secs only going 12mph
14.25X17 3 blade less prop slip but still present, rpm to high and no speed
14.25x19 3 blade had a tiny bit of prop slip, almost compared to the 14.5X19 but just doesn't.
So need to beat the 14.5X19 or buy the same prop as a spare.
WOT loaded with a full load in my eyes.....3 secs to plane, 12 secs to 5700rpm 39mph, 4000rpm 25mph cruise
WOT empty 6000rpm which is max rpm 42mph, 4000rpm 27mph
Seeing the difference between the 14.25 and the 14.5 almost makes me want to try a 15x19.
I may just have to live with the 3 secs to plane though It's not that my bow ever points to the moon like some boats.
4 blade props pop the rear up quickly but then went no where...... 6000rpm in 6 secs only going 12mph
14.25X17 3 blade less prop slip but still present, rpm to high and no speed
14.25x19 3 blade had a tiny bit of prop slip, almost compared to the 14.5X19 but just doesn't.
So need to beat the 14.5X19 or buy the same prop as a spare.
WOT loaded with a full load in my eyes.....3 secs to plane, 12 secs to 5700rpm 39mph, 4000rpm 25mph cruise
WOT empty 6000rpm which is max rpm 42mph, 4000rpm 27mph
Seeing the difference between the 14.25 and the 14.5 almost makes me want to try a 15x19.
I may just have to live with the 3 secs to plane though
It's not that my bow ever points to the moon like some boats.
Fish Assassin
Crew Member
I have tried Merc and Turning Point calculators as well as about 3 or 4 others online. Most tell me to use 15" and 17" pitched props and mostly 13X17 and I know for sure that is wrong.
Id like to stay away from SS as I often distroy a prop every year. Alloy is cheap and easy to replace. Often cheaper then fixing a SS prop.
Turning Point props brag how strong they are. Compairing them selfs to SS props at Alloy prices.
Between having a new motor that these calculators don't recognize and a modified boat unlike many others, finding the perfect prop is a pain in the arse. But it's fun to mess around with and learn.
Everything about my boat has been changed out now except the main portion of the haul and the wipers Some days I feel I may be building a prototype fishing machine for SilverStreak Boats. Besides the lack of suspension seats this boat has everything I could ever want and more......
Id like to stay away from SS as I often distroy a prop every year. Alloy is cheap and easy to replace. Often cheaper then fixing a SS prop.
Turning Point props brag how strong they are. Compairing them selfs to SS props at Alloy prices.
Between having a new motor that these calculators don't recognize and a modified boat unlike many others, finding the perfect prop is a pain in the arse. But it's fun to mess around with and learn.
Everything about my boat has been changed out now except the main portion of the haul and the wipers
Some days I feel I may be building a prototype fishing machine for SilverStreak Boats. Besides the lack of suspension seats this boat has everything I could ever want and more...... 
