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c.1970 15-footer with 70-HP on Jackplate

Posted: Thu Jan 18, 2018 9:36 pm
by SSP
What propeller is best to be put on a c.1970 15-footer with a Yamaha F70 engine and a BOB'S 4-inch jackplate?

I am looking for [a propeller that will provide] all-around performance with good top end.

The boat has a custom center console. Any help will be greatly appreciated. I am sure this has been asked before.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Fri Jan 19, 2018 8:59 am
by Phil T
The recommended propeller by other members for your motor and hull include:

    13 x 16 PowerTech SCD
    13.25 x 14 SCP (6100, 3 holes up)
    13 x 17 Yamaha Painted SS
    13.25 x 14 Yamaha Performance Series

With the setback, you will want the motor raised significantly. The anti-cavitation plate should be 2 inches if not higher above the bottom of the hull.

Remember the WOT engine speed of the F70 is 6300-RPM.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Fri Jan 19, 2018 9:52 am
by jimh
The criteria of "all-around performance" and "good top end" are somewhat hard to define qualities. Generally those qualities are provided by three-blade propellers whose pitch allows the engine to accelerate to the upper one-third of its range of engine speeds for full-throttle operation.

The propellers that Phil mentions vary in pitch from 17 to 14. That seems like a wide range of pitch.

Do you have any data available now about these aspects of your boat:

--present propeller, make, model, diameter, and pitch;
--present boat performance in MPH at various engine speeds in RPM in the range of 2000- to 6300-RPM;
--gear ratio of your engine;
--total weight of your boat, engine, passengers, and gear?

Those data will help us estimate the propeller pitch and potential boat speed that will result.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Fri Jan 19, 2018 12:28 pm
by Phil T
As we know, propeller sizing is not universal due to the different blade design shapes.

If you look at the two Yamaha props mentioned above (painted SS and shiny) the blade shape and rake are not the same at all.

These recommendations come from members actual performance results that have been posted and are not general preferences.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Fri Jan 19, 2018 1:59 pm
by SSP
Thanks guys! I just bought the boat less than a week ago and only ran it for 10 mins the day of to be sure it ran with no problems so I don’t know the boat real well. It has 19 pitch four blade... I don’t know all the specs but will look tonight. It only hit 4600 rpms... I would think it was running probably 32-33 mph with 2 adults and 2 four year olds. That’s what I have so far.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Fri Jan 19, 2018 2:28 pm
by jimh
A small and very light boat like a Boston Whaler SPORT 15--and particularly the early hulls that are quite light--will be very sensitive to crew weight. Adding four people to the boat probably doubles its weight. Any sort of propeller testing should be done with a consistent crew weight aboard, and also with the crew weight you expect to be the most typical.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Fri Jan 19, 2018 7:32 pm
by SSP
So this is what is currently on it:



SOLAS 13x19 4 blade

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Fri Jan 19, 2018 8:56 pm
by El Rollo
I have a 1988 Boston Whaler 15 footer that I customized a little for my needs. It is powered by a Yamaha F70 with 5-inch Bobs Machine manual jack-plate. I have played around with propellers. Tomorrow I will look at the specific brands, pitches, and diameters , and provide some accurate information regarding my personal experiences. You will be VERY happy once you get your rig dialed in for your specific needs.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Mon Jan 22, 2018 12:38 pm
by El Rollo
You are over-propped for your set-up. I made the same mistake when I re-powered from a Yamaha 70 two-stroke engine. The three-blade propeller that ended up working best for me was was a Yamaha Performance Series 13 1/4" x 16" ( MAR-GYT3B-V4-16)

Keep in mind I added a significant additional weight to my !988 Boston Whaler 15. I have a 24-Volt trolling motor on the bow, three Lifeline AGM batteries at 26 lbs each, a substantial bench seat, and a custom-built fiberglass center console. I list these items so that you can compare my set-up to yours in terms of added weight. Classic 15-footers are weight sensitive, so your rig may end up being lighter and may perform better with a different propeller.

Weight distribution is a big factor in the 15-foot model. The way I laid out mine has a lot of weight towards the stern--typically not desirable. But I did this by design.

I ended up with a Turbo four-bladed 13-1/4 x 14 propeller. The model number has changed since I purchased the propeller. I believe when Yamaha purchased the Turbo Brand, the model of my propeller was changed to FX4. On my original box the item is listed as a Turbo Flats Xm so being a a 4-blade, the new name FX4 seems reasonable. It's the same proppeller

Because of the the additional weight in my boat and its distribution towards the stern, I saw a significant amount of stern lift with the four-blade over the three-blade. The three-blade was faster, but that was the only benefit I saw over the four-blade. With the four-blade, I see much better acceleration from a dead stop, much better mid-range, and better low-speed maneuverability. The stern lifting characteristics of the four-blade make my boat ride so much much better it's amazing.

I am assuming that your boat will be set-up more traditionally and may not need the stern lifting of a four-blade. I think the Yamaha Performance Series 13-1/4 x 16 I mentioned or something similar in diameter and pitch would be a good starting point.


At wide open throttle, I can hit the rev-limiter on the engine if I trim it out running solo, so this tells me you may be able to go up in pitch a little if your boat doesn't have all the extra weight mine has.

And just for reference, that four-blade I chose works amazingly when the boat is loaded down. I've have two adults and three kids in the boat with a full day's worth of food, drinks, water toys, and a 10-foot x 10-foot shade tent, and the boat was able to get up on plane without a problem. Keep in mind we were over capacity and the water was flat-glass. It shows how good that engine and propeller combo works.

Hope this helps, and feel free to ask me any questions.

And get that 19 propeller off there as soon as possible, especially If the engine is new. I was scolded for trying to break-in a motor with the wrong propeller. You don't want to lug-down a new engine during break-in.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Tue Jan 23, 2018 11:28 am
by SSP
I spoke to a gentleman at Power Tech and he surprised me by recommending two other propellers over the SCD ["stern-lifting semi-cleaver"]. [The recommendations were for] SWW ["Excellent grip for engine elevation and cavitation-prone applications"] (more of a flats prop he stated) and the NRS ["25-degree rake, good lift, extensive cup, similar to TURBO"] in the 13 x 14/15/16 pitch. I am leaning toward trying the 13 x16 NRS over the SCD at the moment after talking to Marco, however, my gut is telling me to go with the SCD. I think what matters most to me is top end in a healthy RPM range followed by [acceleration from a standing start].

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Tue Jan 23, 2018 12:15 pm
by El Rollo
A three-blade propeller will get you the best top-end speed for sure. Something in 16-inch pitch will be a good starting point.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Sun Feb 18, 2018 12:41 pm
by SSP
I finally got around to trying the Power Tech three-blade SCD ["stern-lifting semi-cleaver"] 13 x 16 propeller after the jack plate was installed. [The propeller] did everything everyone said it was going to do: great top end; 40-MPH without even lifting the plate; slipped a little getting on top; and rev’d 6300 to 6400-RPM. But definitely not my propeller.

I already shipped the propeller back to Power Tech yesterday. On Monday morning I will talk to Marcus about which propeller I want to try next.

What do you guys recommend from here?

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Mon Feb 19, 2018 9:13 am
by jimh
I am following the narrative. I see that the propeller manufacturer did not recommend the SCD (semi-cleaver stern-lifting) propeller, but your gut told you to try it. You tried the SCD; you found it unsatisfactory and returned it. If you want to try another propeller from PowerTech, perhaps you should try one of the models they originally recommended to you in lieu of the SCD you choose.

Not being familiar with the PowerTech line, their very many models, and their three-letter designators, I was having trouble following the narrative with just the three-letter model designators as descriptors for the propellers. I searched the PowerTech website to find their descriptions for their propellers, and appended them parenthetically to your narrative when you mentioned a particular model only by its three-letter designator. I suspect that I am not the only reader who does not have the PowerTech line memorized for propeller model three-letter code and the description of the propeller.

Re: c.1970 15-footer with 70-HP on Jackplate

Posted: Thu Jun 21, 2018 7:07 am
by flymo
Sorry for the delay, just saw your PM. I have not tried the SCD3 in the 17 pitch yet, only the 16. I plan to try the 17 soon - the 16 is wonderful but it will hit the 6450 RPM rev limiter at about 42 MPH when I'm running light and fast. I do not have a jack plate (tried it, didn't like it) so if you're running one you'll certainly need more pitch.

The one thing I would tell you to avoid is a large blade area, heavily cupped design. I tried several with no success - miserable acceleration in the midrange. The Turbo 1, in particular, was terrible with the F70 - I even tried dropping down to a 13-pitch to try and improve the acceleration, and it didn't help. Don't get me wrong - the Turbo is a great prop, just not right for the F70. The SCD3 was a revelation; it brought back the feel of my old Johnson 70 - when you're running around 4000 and floor it, it just scoots. I am hoping to keep that feeling, but stay off the rev limiter, and maybe gain a little top speed with the 17 pitch.

I am convinced that a prop with smaller blade area is the key for the F70 on the 15, when set up as a fast runner. You want to let it spin in the middle RPMs, even putting up with some slip, to let it come up to the speed where it makes good power. The SCD3 works great for me, and I note Turbo props makes the "Quest" model which they advertise specifically for the F70 - might be a good one to try.

As mentioned, I will be trying a 17-pitch SCD3 and will swap back to to my previous 16 if I lose that midrange punch. I confess I'm a little stumped as to why the SCD3 is "not your prop" if it did everything you expected it to - although clearly 16 is not the right pitch for your setup.