Captain PartyMarty
Crew Member
I hear ya guy's and thanks for the offer Ship Happen. I am going to move the engines up first in the next week or so. I will definitely test a four blade.
Trim Tabs and Rough water Operation
- Thread starter Captain PartyMarty
- Start date
By podding you have adversely affect the vessel fore-aft center of gravity & your symptoms fit. The value used to calculate this is "foot pounds"; if you move 600 pounds of engines/brackets to the rear by 3 feet you have affected the COG by 1800 foot pounds This is an example & your actual weight/distance values may differ. There is good reason the boat mfgr placed the fuel tanks more forward on the podded boats. If your boat is 25 feet long, the fore-aft COG should be about 1/3 distance from the stern; let's say 8 feet from the stern. Using the above values, around 150 lb in the bow SHOULD restore the proper COG but with weight being at the extreme ends of the boat instead of where it should be (more towards the fore-aft center), the boat will never perform ideally.
ab1752
Crew Member
So if 150lbs in the bow isn't possible due to structural design, but there is room for additional ballast closer to the original cog, is it straight line math to increase the blast weight required? Or are you suggesting that some boats are simply not candidates for a pod installation due to design limitations? Thx
Yes to both however I am only aware of the issues & math involved; I have owned 2 podded boats, so my experience is very limited. Also, trading an I/O for a podded O/B greatly reduces the overall boat weight which gets more hull out of the water (seems to be a very good thing). Did a 24 Searay back in 1989; chevy V8 to Yamaha 250; no there changes needed & back then I wass too young & stupid to know any better; the shop had some experience in this area plus the pod mfgr (armstrong) was familiar with podding a 24 Searay. But if you do the math on the above example it is pretty good. The other issue is how well balanced a boat is from the factory before modifications. I am sure they all do some math, but how many do the math, test/tweak versus "close enough works for me"? I tried to find info on what stability tests the CG requires but struck-out.
Most recent is swapping twin IB Diesels amidship for 2 300 etec's on a 33 ft Chris Craft. Moved the fuel tanks forward about 8 feet. Boat was originally twin Chevy V8's so when podded was already about 600 - 700 pounds overweight with the diesels. Very little experience with sea trials as I installed a new dash panel/extensive re-wire, but the boat sits 3-4 inches higher & the angle it sits at in the water look good. Need a run to WCVI (from Everett WA) to fully evaluate. Went from steel to AL fuel tanks; overall I am probably 1200 lb or so lighter.
Before the last project I joined boatdiesel.com to use their prop calculator to model weight, horsepower & prop changes to predict performance.
Adding weight reduces MPG & speed so going with little/ no ballast is preferable.
Rain City
Crew Member
What does the CC weigh? Sounds cool. I'm seriously considering this. Or just buying a new boat. The Commander has the same issues. Designed for gassers so sits butt heavy with the diesels.
Before the mod's I guessed at 15K lb. You are I/O with 6 cylinder Volvo's right? My pod is 4 feet long from the stern, & you would loose 800 or so pounds but 6 or so feet farther back. Boat goes 40 MPH with rough bottom paint & with direct injection I get 8mpg at hull speed. You COULD loose that weight by going with Volvo D3's. The conversion was VERY expensive & since my wife is not a forum member I will admit to around $80K US including $30K for used engines. With I/O you have many fewer holes to plug-up & the transom may not need re-enforcing.
My main issue was the shops in my area that do diesels are total A-holes plus low maintenance of O/B power. My surveyor & a repair guy from a diesel shop both told be O/B's were the best choice. Having the props 6 feet back from the stern will be a bit of a pain for DR fishing.
Rain City
Crew Member
I would never consider a diesel IO again.
Doing the math 2600 lb of diesels 8 feet back from COG = 20800 foot/pounds
950 lb of O/B 14 feet back from COG - 13300 foot pounds
Using 1600 lb of original gasser's 8 feet back from COG = 12800 foot/pound.
Looks like podded O/B's would come very close to restoring your boat's original COG (1/3 the total stern-bow length as measured from the stern.
BTW nice job on bucking the vocal minority on the Mark Selective Fishery stuff; thanks. I am a retired Computer Consultant & witnessed great amounts of "cherry picking" facts; seems like just about everybody does it these days.
950 lb of O/B 14 feet back from COG - 13300 foot pounds
Using 1600 lb of original gasser's 8 feet back from COG = 12800 foot/pound.
Looks like podded O/B's would come very close to restoring your boat's original COG (1/3 the total stern-bow length as measured from the stern.
Makes me feel better about the $80K; thanks. Plus the emissions are a huge issue; I want to take the grandkid's & sleep at night.
BTW nice job on bucking the vocal minority on the Mark Selective Fishery stuff; thanks. I am a retired Computer Consultant & witnessed great amounts of "cherry picking" facts; seems like just about everybody does it these days.
Rain City
Crew Member
Well I don't know if I want to claim that prize necessarily but I would like to make an informed decision. It's just too bad you're team black or team white these days. There's a huge area in the middle to be a part of that unfortunately doesn't get you any pats on the back.
Since you asked about Trim Tabs and rough water; here is a tip I learned for certain nasty following seas. I do my best to avoid going out in or getting caught out in nasty sea states, - getting too old and soft for that **** now. However once I was caught out in Sooke Inlet and going towards the end of the Spit to round it into Sooke Harbour. I was going slow and the waves which were very nasty and very close together were hitting us in the butt and were wanting to turn the boat sideways into the troughs where the next wave could hit us broad side. Not a good situation. After doing some research I was later able to work out what was causing this. I had one of the tabs down a bit to balance out side to side weight imbalance that day, but of course at slow speed tabs don't work. What was happening was that the push of the wave was grabing onto that one tab that was down on the corner of the boat and using it to turn the boat somewhat Like a cop using the Pit Maneuver to push a vehicle in the rear corner and spin it out. Now if I find myself in that situation I keep both tabs up, flat and balanced.
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Captain PartyMarty
Crew Member
I picked up some test props from Inlet this afternoon - Revolution 4 17P going to try them out this weekend after I finish raising the motors on Saturday. We will see how much difference it will make
Captain PartyMarty
Crew Member
I was a bit curious about the COG calculation so I took some measurements and looked at the relative shift in CG
It looks like by podding I have shifted the COG by about 11in past the transom. while a New Seasport with a similar setup but the mid ship fuel tank has COG is 8in in front of the transom. I also looked back at the COG of the original diesel engine and its 24 in, in front of the transom. Its pretty clear that moving the fuel tank to the center of the boat has its advantages and that's why Seasport likely made that move. It also looks like you where about right, I would need to put 160LB of ballast in the front of the boat to bring the COG back to where a new seaport would have its COG.
Pretty interesting.
My boat was poded by someone who must have know what they were doing and we do not have a problem with COG having been shifted back. The big 6cyl 225 Opti is a lot lighter than the original inboard-outboard. Further the pod is the large hull forming floatation type which correctly counters the weight of the motor on the pod. I was told some lead has been added into the pod to counter act some of the floation which can be removed if the light two stroke outboard were to be replaced with a heavier 4 stroke outboard. In addition the outboard has a stainless 4 blade prop which adds a degree of stern lift on plane. The large fuel tank is mid ship and slightly forward on the boat and long and narrow along the keel line. I top up the fuel tank frequently and try to not to let it it get below 60% full which adds ballast low down and forward on the boat. With this set up the boat jumps onto plane and the bow comes down quickly and the boat naturally sits flat on the water, both at speed and going slow.
Aces
Well-Known Member
The big thing is moving the fuel tank amidships. We set my 450 L tank amidships as well and the boat is beautifully balanced
Bennett trim tab control switch ,New
new in the package ,,,euro style rocker control switch ...screws and connectors inc..will work with all bennett trim systems..
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Anyone need this? Not mine.
searun
Well-Known Member
I didn't have time to review all the posts, so if this has been suggested, please ignore. I podded my last boat and immediately had exactly the same problem. Trim and tabs would not address the problem. So, a cheap and immediate fix. Install a set of those dolfins...immediately solved the problem and really made a huge difference in keeping the bow down in rough seas.
Captain PartyMarty
Crew Member
Haven’t tested yet might be doing some testing on the weekend
Captain PartyMarty
Crew Member
Finally managed to get out and test the 4 Blade today. Here are the plotted results of the 4 blade and the 3 blade under the same conditions. I would consider the loading light with approximately 50% fuel and 2 People On-board.
Observations from my side.
1) I need to move up 1 or two Pitch sizes on the 4 Blade As I was able to go over the 5400 RPM limit for the SeaPro's. I hit 5520 rpm and It didn't feel like it was done there I backed off pretty quickly.
2) The 4 blade does fix a lot of issues, The boat didn't porpoise at all and so I didn't need to use any trim tab to control the boat
3) As shown in plot 2 I was on plane at much lower speed with the 4 blade.
4) The boat overall handled much nicer with the 4 blades.
Next Steps
1) See of I can get an 18P Revolution 4 prop setup
2) Move the engines up 1 hole.
Observations from my side.
1) I need to move up 1 or two Pitch sizes on the 4 Blade As I was able to go over the 5400 RPM limit for the SeaPro's. I hit 5520 rpm and It didn't feel like it was done there I backed off pretty quickly.
2) The 4 blade does fix a lot of issues, The boat didn't porpoise at all and so I didn't need to use any trim tab to control the boat
3) As shown in plot 2 I was on plane at much lower speed with the 4 blade.
4) The boat overall handled much nicer with the 4 blades.
Next Steps
1) See of I can get an 18P Revolution 4 prop setup
2) Move the engines up 1 hole.
brutus
Well-Known Member
4 blade is the way to go, once you get the pitch tune in you will love them
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