Pitch problems

[McShip01]

I have a 2002 15ft seapro. weighs around 1500lbs loaded. I just put a 2012 yamaha f70 on it. With a prop 13.75/15pitch. I can't get it above 5000rpm without it porpoising. I tried a 13.75/13pitch that got me to 6200rpm but wouldn't plan out till 4500rpm and only got 30mph. I know this boat should get close to 40mph. Should I try dropping the diameter to 12inch/15pitch or try a four blade

[HydroliftErik]

30mph is sounds pretty good, that means you have a slip approximately 5%
thats very good for a "standard" rigged boat.

so the grip of the propeller isnīt a problem, therefore there is no reason to go for a
4 blade,

try a 12" diameter with 14-15" pitch, maybe a stainless steel?

this engine needs to rev alot to get the maximum power out of it.

[McShip01]

Thanks for you input. Ill give that a shot. I'm adding trim tabs this weekend. Would that help the issue at all? It had a 2002 four stroke 50hp Honda that ran 27mph that is why I figured I should get more speed. Boat max is rated for a 70

[Capt. Barry]

Try this?

Did you try triming (tilting) the motor down a bit? If the trim is too high, any boat will porpoise.

Just for info. I have a 170CC Triumph. Boat with motor weighs about 1,500. The motor is a Yamaha 2 stroke 70hp TLR. With the original 13 1/2 X 15 prop, rpm would top out about 6,100 rpm (motor is rated between 5,000 and 6,000). With a 13 1/4 X 17 prop, RPM would top out at 5,500 rpm. Both props are aluminum. Just broke the "17" prop this past monday hitting a rock. Going to look at a stainless steel prop. I think a 13 1/4 X 16 would be perfect. I know Yamaha makes one, just have to save some dollars.

[McShip01]

With the 13 pitch I'd start out all the way trimmed down to get out of the hole. At 3500rpm I'm still pushing water. Down to 4000rpm, I start to hit plane. tilt the engine up to a setting 3 after I get out of the hole. Put it down to full throttle, raise it to 3 1/2, and nothing more or it will porpoise. I get 30mph at 6200 rpm. With 2 people.
The 15 pitch gets on plane at 16mph/3000-3500rpm. Tilting the engine up to 3 again. At 4600 it picks up a little speed (24mph). Raise it to 3 1/2 engine starts to rev more and goes to 5100(fast) and it starts to porpoise. Even had 180lb person sitting on the bow.
All this was tested in lake conditions.

Would trim tabs control the porposing?

[McShip01]

The cavitation is dead even with the deadrise. I have 2 holes I could lower the engine..just another thought. I'm gonna work on it this weekend. Ill let you know how it goes. I appreciate the help.

[Capt. Barry]

Lowering the motor or installing trim tabs introduces drag which slow the boat down. For low speed, trim tabs will get you on plane faster.

From the boats I've been on, all of them will propoise if the motor is trimed up too far.

Put a piece of tape over the trim gage, forget about the numbers for a moment.

Start with the motor slightly trimmed in, accelerate to WOT, trim out until you begin to porpoise and then trim back in a small bit. Remove the tape from your guage and record your number. Now, if you add weight fore or aft, all numbers will change. Small boats have big change when center of gravity is adjusted.

Hope this helps.

[Bilge Rat]

The prop is trying to lift the bow and then loses grip and drops it. Too much trim or too little power can cause it.

Aluminum props flex and cannot be shaped for best performance. Thy a SS. prop.

Try tucking the motor and push the throttle to WOT when the boat comes out the hole you can throttle back. As the boat planes out trim it up a little for best running.

A SS. prop that has a little more stern lift may help your setup get on plane and reduce porposing at midrange. Prop design is more than pitch and dia. You can get more stern lift or bow lift props to matchup you hull and motor combo.
SS props can be designed to meet any need Aluminum can't.