more prop?

[spete]

Have an 18' flats boat with V bow which tapers to a rather flat bottom in the back with a tunnel. Gulf coast is a bit like Whaler/McKee Craft with the swept bow. Running a 115hp with jackplate and it currently has a 4-blade 16x13 that looks like it's been re-worked a bit. I measured across blades and get 12 1/2 inches. So i guess a more accurate description of the prop is a 16 x 12 1/2. Power and hole shot is great and throttle response is very strong at all ranges. Looking to get more top end since it seems to have such good power and easily reaches the 5500-5600 WOT. Should i look at 3 blade or 4 blade and how much should i try proppin up? I heard that the 4-blade is going to be better for running with the motor jacked up/ have less blow out. Wondering if i just go up to the actual 16x13 size and go for a bit larger diameter and same pitch or higher pitch and same diameter or some other combination? Boat specs indicate a displacement of 1200lbs/92' beam/17-7" length. Will currently do about 35-36 mph at WOT and cruise at about 30 for 4900 rpm. Thanks for your reponse!

 

[ddaigle]

Re: more prop?

Hey spete, Lots of tunnel boats down here in the Laguna madre. If you dont go real shallow like we do here(12 in. or less) you could probably go up a couple of sizes in pitch. But be careful because you will hurt your shallow takeoff performance a lot. Also, bay tunnel hulls have a peculiar aspect of the faster you try to go the more they try to lift the back and push the front down which can cause its own problems. Low 40's mph seems to be about max for these boats. Boats here with small motors run almost as fast as the megamotors because of this effect. A lot depends on your tunnel design but be careful. A guy I know had a merc 150 on his tunnel and at speed he found the front end wanted to plow and he would eat a lot of waves over the front. Not fun or safe.

 

[ob]

Re: more prop?

If your primary intention is to gain top end performance ,then the three blade is the way to go.You could increase pitch by 2 or more in.<br /><br />Since that boats hull style is an easy planing design ,the four blade is not necessarily needed.However ,if you frequently run in shallows and use the jack plate to its maximum benefit,you might just stick with the four blade that you have.Your WOT rpms look good .

 

[Dhadley]

Re: more prop?

A couple of things. I think you have the diameter and pitch reversed. In prop sizes the diameter comes first then (x) the pitch. No biggie, just so we're all on the same page.<br /><br />Going up in pitch from your 16" (correct me if thats not right) will cause you to lose top rpm. Youre lacking on rpm now. Going up in pitch will lug the motor even more than it is now. Thats not good. <br /><br />Since the diameter measures out at -1/2" that means each blade is 1/4" shorter than its supposed to be. The tip of the blade is where a lot of technology is. My guess would be that if we played with the height to gain rpm, we're going to start venting in tight turns. I would still raise the motor but thats a guess at what we'll find. Its on a jackplate so it should be easy to raise. <br /><br />Raise the motor a 1/2" or so and see what happens. We are only looking at straight line top rpm's. Dont worry about venting in turns or on take off. We want to see how the motor reacts to height on that particular hull. Watch your water pressure.<br /><br />Good luck!

 

[spete]

Re: more prop?

Thanks for the responses ... <br />I know this hull probably isnt' built for speed but on those long hauls down the ICW i'd' like to get a little bit higher cruising speed in the 4900-5000 range. My yamaha 115 is spec'd for 5500 WOT and the prop hub is stamped 16x13 for some strange reason - i assumed that it's 16 pitch and 13 dia. because the distance measured across is 12 1/2. Unless there is some way to trim down a 16" prop to a 12 1/2? I used the prop slip formula for a 16 pitch and it seemed to work out about right as opposed to the other way around. I definitely want to keep the good hole shot and probalby a 4-blade for shallow gittin up and running in the back lakes of ESB. Maybe just try the 4-blade in a 13x17 if there is such a beast?

 

[ob]

Re: more prop?

Powertech makes a 13 1/4 X 17 4 blade cupped prop that is listed for a 115 Yamaha.<br /><br />Turbo also makes one that is called a Turbo Flats IV prop that is also 13 1/4 X 17.

 

[spete]

Re: more prop?

Thanks OB ... any experience/preference between the Power tech and the Turbo? Turbo Flats IV sounds like it might cater to shallow water - may have blade/cupping characteristics with more bite out of the hole in skinny H2O? Might be just the ticket to tweak the cruising mph up a bit. Thanks a bunch for the suggestions!

 

[ob]

Re: more prop?

If cost is not an issue,the Flats IV would meet your application needs.It runs around 100 clams more than the Powertech.