Prop ?

[shuswap outdoors]

Hey guys, I'm pretty new to boating so I might be asking a stupid question....

I have a 16' Lund SSV with a 30 Merc on it with a mercy spitfire 9.3x11p..... Solo and with 2-3 people to boat runs great (gets on step fast and runs in the 25-29mph range) but the few times I have 4 people or 3 with a lot of gear the motor lags and am only getting 21mph and the motor never sounds like it's reaching a higher rpm.

So I am wondering if I picked up a second prop (to have a spare) in the 9.3x9.5p to run when I am loaded down heavy. Would I get a little more speed out of the prop as it would be spinning faster? Would it get on step faster?

I have tried using one of the spark-plug wire RPM gauges with zero luck getting an accurate reading, so I have no idea what I'm at. Is there a better way to find out my RPM?


Thanks!

 

[ericl]

What you NEED to do to solve this problem is to measure RPM at WOT with the light load & prop it to hit the high end of the published WOT RPM range. Then see if with a heavy load the same prop at WOT will fall into the published range.

So basically you have 3 options:

Spend money
live with it
buy a second "smaller' prop.

 

[shuswap outdoors]

Thanks for the info. Can you recommend a decent tach?

 

[bigdogeh]

What you NEED to do to solve this problem is to measure RPM at WOT with the light load & prop it to hit the high end of the published WOT RPM range. Then see if with a heavy load the same prop at WOT will fall into the published range.

So basically you have 3 options:

Spend money
live with it
buy a second "smaller' prop.

except I wouldn't recommend living with it if you run loaded quite often and can't reach maximum rpm's. That can be pretty hard on an engine. sounds like that may be what you have been doing every once in awhile. (kinda like going up a steep grade in overdrive...)
Going down a pitch or 2 in prop pitch should help when your loaded up. As ericl mentioned you want to be able to reach max rpm the engine is rated for when loaded. If you can't you can have premature engine failure if left too long.

I'd look for an in-dash tach, but you could purchase a photo tachometer gun on ebay fairly cheap and remove the cover on your engine. put a piece of reflective tape on the flywheel and check rpm that way. Just make sure you have someone checking the rpm for you while someone is steering and watching out in front of the boat.

http://www.ebay.com/itm/DIGITAL-NON...263983?hash=item41b77c7fef:g:Fr4AAOSw241YmuoB

 

[kaelc]

A bit of overkill but you could get your engine data on your lowrance GPS and maybe others like Garmin. I have it on my Yamaha 115. I believe you need an HDS and they are not compatible/easy for all motors but lots of second hand ones coming up for sale now. http://www.lowrance.com/en-US/Products/NMEA-2000/Mercury-VesselView-Link.aspx

A possible fix may be a Hydrafoil fin or trim tabs to help get and keep you on step with a load. I replaced the generic dolphin wings with an SE300 and my boat performs way better. For less then $100 I was really impressed how it fixed proposing, buy locally but here is one of the only links I could find. https://www.westmarine.com/buy/sport-marine-technologies--se-sport-200-hydrofoil-black--4707741

 

[shuswap outdoors]

thanks guys

 

[ericl]

Yeah. living with it is bad except it's a Merc - AKA "Black Death". Sooner it dies the better. Back in the 60's the saying was if you wanna get there first get a Merc. If you wanna get back get a Johnson.

 

[shuswap outdoors]

hahaha....... yeah not a big fan of it but it came with the boat when i bought it.... Would love to swap it out for a 40 or 50 Yamaha or even a Honada, their weights are pretty impressive but hard to find reviews on them. Also convincing the wife that I should spend more money on the boat is going to be a tough sell.

 

[SteelyDan]

Just get a prop 2" lower in pitch. Your top speed will not increase. It will deacrease but loaded will plane faster and cruise at a lower rpm.

 

[shuswap outdoors]

thanks!