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Author Topic:   Right Prop for 15 w/70hp?
SSCH posted 06-26-2002 09:48 PM ET (US)   Profile for SSCH   Send Email to SSCH  
I've just taken delivery of a new 15 Alert (old style hull, like the Super Sport but a little heavier) with a 2002 Yamaha 70 hp (2 cycle). The boat will have a custom wooden interior that should add not too much weight. The engine will be mounted 5 inches back on a Bob's manual jackplate, probably up about 3 inches from the normal depth.

Any great ideas on which prop to try first? I'm interested in top speed obviously, but also intend to fish the boat in shallow water, so need a good hole shot too. I'm willing to buy more than one prop if need be.

jimh posted 06-26-2002 11:23 PM ET (US)     Profile for jimh  Send Email to jimh     
According to Whaler's published results, with a 70-HP engine the 15-foot hull was capable of speeds of 42 MPH. Your hull may weigh more, so perhaps slightly less speed is possible. This information is included in my article on the 15-foot hull (see REFERENCE section).

Using my on-line PROPELLER CALCULATOR (also in REFERENCE section) and using your engine's ratings of 2.33:1 gear ratio and max RPM of 5500, you can find a prop pitch that will to get to 42-MPH at wide-open-throttle. Using a factor of 8% SLIP, this computes to 20.4-inches of pitch.

Because often props are made only in odd-inch increments, and because they are often cupped designs which makes their effective pitch slightly larger, 19-inch pitch prop may be indicated.

I believe the stock prop on a Yamaha 70 2-stroke is probably a 17-inch pitch. I would try that first. Assuming you can turn that prop to max RPM, the propeller calculator shows you going about 35 MPH with that set up. I would think you will have no problem doing that--I can go that fast with my 50-HP.

Hmm...maybe that 19-inch pitch is the way to start.

Highwater posted 06-27-2002 07:32 AM ET (US)     Profile for Highwater    
Jim, Thanks for the the calculator. I have not been able to get it to work, but it is probably because I am on an old computer.

In general, if you tend to carry a lot of weight (1000 lbs on a 15'), and if you want to be able to go 20 mph while towing a disabled boat, and if fuel economy is more important than top speed, would you tend to get a prop with a higher pitch (e.g., 21") or a lower pitch (e.g., 16")? Thanks! David.

Bigshot posted 06-27-2002 11:49 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I ran a 19 on my Johnson 70 but not sure what gear ratio differences there are. My 70 also turned 6000rpm's. Try the 19 as Jim says and if you get 5200-5500 keep it. If higher than 5500 you will need a 20 or 21 nad lower than 5200 a 17 would work.
jimh posted 06-27-2002 12:57 PM ET (US)     Profile for jimh  Send Email to jimh     
Sorry to hear the Prop Calculator does not work for you. It should. It does not require any activity or processing from the browser; all the work is done on the server. You just fill in any four of the parameters and click CALCULATE.

If your browser can submit forms, and most any browser since the dawn of HTML can, it should work. Everything is accomplished on the server, which then refreshes the page with the new answer.

It may be a problem with your host/provider and an upstream proxy server that prevents you from seeing the refreshed page.

There are some user interface enhancements which depend on the browser being able to use style sheets. If your browser is confused by style sheets this could be the problem, too. But in most cases a page which uses style sheets can be displayed in a browser that ignores style sheets without problems.

Back to the boat:

The goal of prop choice is to find the largest pitch prop which can be turned to maximum rated RPM by the engine.

If the pitch is too large, the engine speed will be limited . The engine will be lugging and could be damaged.

If the pitch is too small, the engine speed will be in excess of recommended maximum. The engine could be damaged.

If the boat is to be operated with a heavy load most of the time, you will want a prop with a pitch lower than would be used with a light load. In your case that would be the 17-inch pitch prop.

Fuel economy will be determined more by the RPM range you operate in than by prop choice. If you operate near WOT fuel economy will suffer. If you operate at 50-70 percent of WOT, you fuel economy will be much better.

In your case, the prop calculator shows:

RPM = 3800 (i.e, 5500 X 0.70)
Ratio = 2.33
Pitch = 17
Slip = 15% (typical for this situation)
MPH = 22

where2 posted 06-27-2002 06:36 PM ET (US)     Profile for where2  Send Email to where2     
I agree with BS's thoughts. a 13-1/4"x19"SS on my 70Hp Johnson straight bolted to my 15' Sport runs 50mph (truly frighteneing speed when you're that close to the water in such a light boat). I'll have to see whether the Owners Manual has the lower unit gear specs in it... I suspect they are similar to a Yammy. I keep a 17" pitch for pulling slalom skiers, the 19" is a drag on a slalom, literally!
SSCH posted 06-27-2002 07:58 PM ET (US)     Profile for SSCH  Send Email to SSCH     
Thanks all

Jim .... I did use your prop calculator and came up with a 19 or 21 pitch choice. The Yamaha is capable of 6000 rpm according to the Yamaha website. I have yet to see the engine or any of its literature. The boat looks great (for its current bare hull status).

I'm inclined to try the 19" and hope that I can step up to the 21". I may try a four blade 20" performance prop. I'll post the results after I build the interior, get the motor mounted, splash the boat, and break in the motor. Don't anyone hold their breath. I want to relish this project.

Hopefully, my dealer has a bunch of spare props to experiment with, or I'll run out of money for this quickly using SS props.

PMUCCIOLO posted 12-17-2002 03:14 AM ET (US)     Profile for PMUCCIOLO    

Do you have any updates for us regarding the performance of your CPD 15 with the 70HP Yamaha?

VMG posted 12-21-2002 05:05 AM ET (US)     Profile for VMG  Send Email to VMG     
Jim's (and others) advice is right in line with what I saw working for my 15SS CC w/ Johnson 70. Actually tried 3 -- a cupped 19, the same prop in 21, and a non-OEM uncupped 21. The cupped 19 and the uncupped 21 ran about the same except for hole shot. WOT, the 19 ran a couple hundred rpm over the limit with just me aboard. But given the activity planned (lotsa skiing, tubing etc) I went with the 19. Light loaded at WOT, the cupped 21 would run within 100 rpm of the limit, but the hole shot was unacceptable and it was actually TOO fast at top end ;).
jodieell posted 06-09-2006 11:02 AM ET (US)     Profile for jodieell  Send Email to jodieell     

Funny thing, I was about to ask the same question. I have a little 15' Renegade Nomad flats boat currently powered with a 60 HP 4 Stroke EFI Merc. This is a relatively light boat, glass, has a 13 degree deadrise at the transom. With me alone, 9 gals of fuel, no water in the well, the boat skips right along at a GPS speed of 37.4 SMPH in salt water with engine maxing out at 5800 RPMS turning 14" pitch, 3 blade SS prop. Hole shot is OK until you load up the boat with a chubby fishing buddy and the bait well (in the rear) full of water.

I'm thinking of repowering with a Yamaha 70 HP 2 Stroke. (Why not a 90, you ask?) Would like to, but that would be over the max power rating of 75 HP for this little boat. Anyway, the question is this to anyone who can offer a educated reply: We can certainly assume the hole stot will be better. But how much will the Yamaha outperform my merc at top end turning, say a 17" SS prop at 5500 to 5800 RPM? I would guess the boat might run about 43 to 44 SMPH. Any comments?

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