upgrade my prop on homemade speed boat. Help Please

[tmlp103]

Hello,

I built a 11ft TNT Glen L boat a while back and bought a 1982 35hp Johnson long shaft for it because that was all I could afford at the time. The boat is technically rated for 50hp. I wound up having to put a massive jacking plate on the back and I even had to add wood shims between the jacking plate and transom mount to get the boat to plane out. My top speed was about 28mph well under what I was hoping for. Well I have some more money now and I am look at a 1994 40hp Johnson 15" shaft. My goal is to remove the jacking plate and to get as much speed as I can out of this new motor.

My questions are is there anything I can upgrade inside the motor itself to get more speed? And my other question is what would be the best prop for the motor? My ultimate goal is to get this thing to really haul on the water




Thanks for any info in advance,
Tom

 

[Bondo]

And my other question is what would be the best prop for the motor?

Ayuh,..... Welcome Aboard,...... Ya gotta splash the boat, 'n see how it handles, before ya fine tune the prop,.....

Ya need the pitch that puts the motor near the top of the rpm band,.....

After ya run it, you can decide whether the hull needs more bow lift, or more stern lift,....

Then pick the prop style best suited, in the pitch necessary,.....

 

[Sea Rider]

Need to tach that OB and check max wot rpm, check if inside factory specs, ideal is to dial a prop or play with pitch to rev around max wot numbers, will have good hole shot and nice top end speed. Engine height is other important issue to have into considerationh, assume you will be boating with just 2 up being a two seater, right ?

Happy Boating

 

[steelespike]

Something is drastically wrong with the 35 setup; speed with a 15" prop should be about 38 mph. @ 5800 rpm.
I think that a 21" pitch is a starting point for the 50.Speed at 5500 rpm should be about 40 mph.Not sure if you can find a 21"
You should keep the jack plate for fine tuning the setup.You will ultimately need a refined prop.You will need a tach,
Only way to know how you're really doing.Check out the induction tachs.

 

[tmlp103]

so you think I shouldnt buy a 40hp short shaft and remove the jacking plate?

 

[Bondo]

so you think I shouldnt buy a 40hp short shaft and remove the jacking plate?

Ayuh,..... I believe what's been said is to buy the right motor, 'n still keep the jack-plate,....

 

[steelespike]

For some reason after you mentioned rated for a 50 It stuck in my head.
The 35 has a 1.93 gear ratio The 40 and 50 are 2.42.
The 2.42 may need a higher than normal pitch to squeeze out more speed.
It does make sense to use a 15" motor regardless what hp.And a jack plate
will make it easier to fine tune the height.
A really fast setup may struggle to make plane and a jack plate could help adjust the ideal height.

 

[tmlp103]

For some reason after you mentioned rated for a 50 It stuck in my head.
The 35 has a 1.93 gear ratio The 40 and 50 are 2.42.
The 2.42 may need a higher than normal pitch to squeeze out more speed.
It does make sense to use a 15" motor regardless what hp.And a jack plate
will make it easier to fine tune the height.
A really fast setup may struggle to make plane and a jack plate could help adjust the ideal height.

Okay I will go ahead and buy the motor then and see were am I at from there. Thanks Guys for the help

 

[Sea Rider]

It's a bad idea to buy a slight larger OB without prior testing that 35 with a Tach, you'll be surprised what a right prop maximization can do for your combo. If thar OB is not vevving right to its near full wot rpm, forget all about counting with a labeled 35 OB while performing less, how much less, nobody knows. A tach will 100% tell..

Have not answered if that combo is or will be used for just 2 boaters...

Happy Boating