Top Speed for my boat???

[zach103]

I've been wondering what a good WOT speed i should have with my larson .. it says here on iboats that the boat with an empty tank and no gear/ people is 1250lbs.. i have the 88 Spl Johnson engine on it.. im not sure what prop but i can find out soon..

Right now my top speedometer speeds is about 28.. i just got a gps to do a reg test.

 

[tashasdaddy]

Re: Top Speed for my boat???

zach speed it not what you set the motor up to. you set the motor so that you get optimum RPMS. 88 spl wot rpms, should be around 5800. you do this with the right prop, engine height, and tilt angle. speed is just the results of a properly set up hull and power plant.

 

[JB]

Re: Top Speed for my boat???

A useful formula for estimating theoretical top speed for a planing hull is:

mph=[square root of (HP/weight)] X bottom factor.

The bottom factor is an estimate of water drag on plane, where a flat bottom is 250, a deep vee about 150 and other hulls somewhere between. My 17' Boston Whaler bottom factor calculated to 195.

 

[zach103]

Re: Top Speed for my boat???

so i did two calculations.. first was with the boat weighing 1500.. and i got 50mph..

second was the boat weighing 2000 and i got 29 mph.. both with the 150 bottom factor..

so looks like my goal is too loose some weight on the boat. and would a good tune up help?

 

[ezmobee]

Re: Top Speed for my boat???

Get a tach and GPS. See if you're hitting your target WOT RPM range. I'd expect 35-40 out of that rig.

 

[steelespike]

Re: Top Speed for my boat???

As suggested 35to 40 should be possible.
By all means be sure the motor is in excellent tune with a good sound prop.
Is the bottom clean?Make a run and get us the prop size and the max rpm and speed. Motor height is an easy,free way to improve speed.The antivent plate ,just above the prop, even with the bottom is considered a starting point.Most end up about an inch above.Raise the motor a hole at a time to a point just before venting(cavitation)becomes a problem.When making changes do one at a time so you
evaluate the results.

 

[zach103]

Re: Top Speed for my boat???

When i order a new tach it should come with the appropriate set up for my engine right?

my tach now is pretty messed up.. i will try that and get all the info first.. than ill try adjusting

 

[ezmobee]

Re: Top Speed for my boat???

At least on the tach I bought.....it was already set correctly for a Johnson/Evinrude.

 

[zach103]

Re: Top Speed for my boat???

what one did you purchase?

And how bout the throttle linkage.. should i adjust that?

 

[ezmobee]

Re: Top Speed for my boat???

what one did you purchase?

http://www.iboats.com/Tachometer-7K...2203075--**********.415887066--view_id.686333

Teleflex? Marine Eclipse Series 3'' Tachometer

 

[zach103]

Re: Top Speed for my boat???

Shoulud i adjust the throttle linkage?

 

[zach103]

Re: Top Speed for my boat???

Ok so the boats home for the summer and i looked at the engine and the plate was about 4 inches from the bottom of the boat

 

[ezmobee]

Re: Top Speed for my boat???

Snap a pic. That sounds like it would be ventilating all over the place.

 

[zach103]

Re: Top Speed for my boat???

This is it with the motor almost all the way down.. this is after i moved it down a hole

 

[5150abf]

Re: Top Speed for my boat???

Don't get to wound up chasing speed, as stated, get a tach and set the engine up right, get a manual and do all the normal maintanence items, linc and sync and decarb then see what she will give you and that is all it will do.

Don't just go "adjusting" things like the throttle cable, it is tied to the timing, if you adjust the throttle you adjust the timing, if you don't do it right you can really screw up the engine.

I have been down this road, tried all kinds of stuff, mine goes 29 mph and that is it.

 

[ezmobee]

Re: Top Speed for my boat???

Ok so the boats home for the summer and i looked at the engine and the plate was about 4 inches from the bottom of the boat

Looking at the picture...I'm thinking the 4" you provided was the horizontal measurement. The one we're talking about it vertical. Your motor looks to be in a good position there.

 

[zach103]

Re: Top Speed for my boat???

ok so im digging this thread up i figured since its mine im allowed to lol

so my prop is 13 3/4 x 15

Hows that sound? too small? just right?

Oh and i bought new gauges so ill have the rpms this season