Performance problems with the new Yammy !!!

Ok guys and gals I have a 1997 scout 155 fish that had a 1997 four stroke 50 hp Yamaha on it top end I would get 33 mph according to the Raymarine dragon fly and it planed out decent . I recently put a new 2014 Yamaha 70 hp on it and trying numerous props the best performance I got was only get 36 mph according to the same gps and it does not plane out worth a dam until I put a hydrofoil on . The props I tried 15 x 15 SS to a 15 x 17 SS(2 different brands with different results ) a 15 x 19 aluminum and another SS prop that would hit make RPM’s at half throttle . According to a few mechanics after 10 of them were looking at it some of them suggested my motor was to low so I raised it up and am now as it was explained to me trying out the same props again according to my research the new 70 weighed only about 30 pounds more than the 50 and all props tried were the solas brand and quicksilver brand .I am beyond frustrated with the dealers “here try this one” approach . Here are my questions 1. Is it possible to hit max rpm"s but not fully take advantage of what the motor is capable of ? 2. Going from the 50 to the 70 shouldn’t their be more than a 3 mph increase in speed or what top end speed increase should I be seeing or expecting ? 3. I looked at the Yamaha performance review and according to the test they did on a scout 160 fish with the same motor they were hitting low 40’s top end and that boat weighs about 200 pounds more than mine not counting the extra fuel it held either what could be so off ? 4. Does anybody (like Chris V or Larry or any other boat guru)have suggestions on a good prop to start with or any suggestions on what might be going on or who could run it or look at it or something other than “here just try this one again” because this is going on about six months now and I am running out of patience with this whole deal . Thanks

Do you still have the hydrofoil on it? When I took mine off, I gained 7 mph. Just a thought. I’ll leave the real advice to the experts.

2000 SeaPro 180CC w/ Yammy 115 2 stroke
1966 13’ Boston Whaler w/ Merc 25 4 stroke “Flatty” |
President, Summerville Saltwater Anglers

I took it off once the motor was raised because I was told I really shouldn’t need it . But you are correct it did slow me down as well I just put it on to help plane it out . Some of the props they had me try I wouldn’t even get out of the twenty’s for top end and when I informed them of this one mechanic actually told me “hey that might be all its going to do”

Which prop got you to 6300 rpm at WOT?

“Banana Pants”
Indigo Bay 170
90 Johnson

Wilderness Ride 115

the solas 15x17 ss with top end of 36 mph

the next prop I am trying is a solas 15.75 x 13 any thoughts ?

you need a 17 pitch to run 40 mph on that motor
that doesn’t mean the motor is capable of turning a 17 pitch prop at 6300rpms, it just means that mathematically that’s what it will take
you could have an issue with the hull not trimming out properly, the motor could still be to low on the transom or there could be an issue with the motor not making power

you should be running a 13 1/2 x 16 or 17

this helped me get to a comfy medium…

Fishing Nerd

“you win some, you lose some…but nothing beats getting some!”

Try a 13.5x16 Yamaha SS prop.


First of all thanks for all the responces , That being said from what info I have found the 13.5 x 16 is what I thought I should be running as well but they were very addiment on that not being the correct one . Two more questions will I get better performance out of the same size yamaha brand props versus brand X ? due they help maximize performance of the engine or is it irrelevant on the brand ? Again thanks to all !

mercury makes some of the best performance props on the market, but you also pay the most for them
there isn’t going to be a lot of difference with only 70hp and 40mph top end between the brands when comparing equal size and pitch.
the only exception to that will be a stiletto prop.
a 15 pitch stiletto will run more like a 17 from any other brand, they have a lot more cup, giving you more bow lift

The one thing that hasn’t been mentioned so far is that the boat itself may be causing your limited speed. Some hulls I have seen come straight from the manufacturer with a defect that limits its overall performance. This can also happen to a hull over time if not supported properly on its trailer.

a 15"+ diameter prop is too big for that engine. Like some others suggested, you need to be running something in the 13" range. based on your past performance, you should run about 6 mph faster with 20 more HP

Iain Pelto
Sea Hunt Triton 160 w/ 90 ETEC “JB3”
Native Manta Ray 14

Sent you a PM.

218WA Sailfish
The "Penn"sion Plan