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Author Topic:   Sport 15 / 50 HP Speed at Plane
jimh posted 02-13-2000 01:57 PM ET (US)   Profile for jimh   Send Email to jimh  
My Sport 15 (1976) Whaler is powered by a 50 HP Merc500. It is currently propped with a 10.12 DIA x 15 PITCH 3-blade Merc aluminum prop.

This gives a top speed of about 35 MPH at 5300 RPM. But the boat won't stay on plane below about 3000 RPM / 22 MPH. At that throttle, it is quite touchy, and will fall off plane easily. Also, the engine seems to be right on the verge of lugging--it really wants to run either faster or slower.

I can tell that a previous owner had Dol-fins on the cavitation plate: Do they help with lower speed planning?

Any other 15 Sport owners out there with comparable MPH vs RPM and Prop data?


lhg posted 02-13-2000 03:28 PM ET (US)     Profile for lhg    
Jim: I'll try to help you out here, although I have never owned or have even been out in a 15! Hopefully, Clark Roberts will show up here, and he is the guy who really knows this stuff on smaller Whalers & knows your engine also.

I checked out an old 1976 Percury prop chart that I have, and for your engine, which weighs 185LBS with a 2:1 gear ratio, they indicate a 15" pitch prop will push 1000-1600lbs 28-35mph. They also show a 14" prop @ 1100-1700lb @ 26-32mph. Max rpm for your engine is the usual 5500. Depending what you have in your boat, it could be that a 14" prop would be better, and keep you on plane at a lower speed, but I don't know how a low a speed a 15 will plane. Even my 18 Outrage, with two big engines, will plane at 20mph or lower. Aluminium props represent the low side of prop technology, and quite frankly, I think they're a waste of money except as an emergency spare. You should probably get a 3 blade stainless "Vengeance" model for enhanced performance. There are probably plenty of them around used also. You should also be sure your engine is operating up to specs, and that it is not mounted too low on the transom. With a stainless, non-hi-rake prop, like a Vengeance, the cavitation plate can be about 1/2 above the water when planing and trimmed out properly.

Which brings up your next question, Doel-Fins and other similar devices. Once again, I've never used them, or even considered doing so, but this is one of those items people either swear by, or hate. I'm the latter, and think they really look "tacky" attached to an engine! "Powerboat Reports", a non-advertising supported consumer reports type of periodical, and highly respected in the industry, did an extensive study of these devices about 6 years ago, and concluded they were a waste of money and did nothing to improve the performance of any boat or motor.
All of the engine manufacturers agreed, from their own testing. This does make some sense, since one would suppose that if these things were so great, they would be made an integral part of the engine's cavitation plate. And that is exactly what the MFG's said. And also, if an engine is properly set up a boat, in most cases the cavitation plate is running out of the water when the boat is planing. A Doel-fin running in the air doesn't make much sense to me! I have heard from those that like these things, that they believe they help a "hard-to-plane" boat get going quicker, but this is not a characteristic of Boston Whaler's well designed classic models. To me, it seems like they would be an additional drag, if running submerged. This is a controversial subject, and ought to get some quick action here!! I'm sure others will disagree with me.

jimh posted 02-13-2000 05:10 PM ET (US)     Profile for jimh  Send Email to jimh     

I am in agreement with you on the Dol-fins on two counts:

1) if they were so great why didn't the engine maker put them on there in the first place! and,

2) making the underwater appendage bigger would seem like it would have to add drag.

Hey, maybe that is it: the added drag makes the boat plane slower!

Well, I'll try a different prop next spring.

I am leaning toward a 4-blade composite with less pitch and slightly larger diameter.

Being old sailors, the firstMate and I enjoy cruising around at less than wide-open throttle.

Also, I think I am hitting an RPM range in the engine where it must be right on the edge of opening the carburetor secondaries, since the throttle gets rather touchy at that point. It almost seems like if the engine had a bit more power at that RPM the boat would plane.


eolson posted 02-14-2000 07:19 AM ET (US)     Profile for eolson  Send Email to eolson     
I've got a 1983 Sport 15 Whaler with Johnson 70HP and a Dol-fin. I bought the boat back in October '99, and it came with the foil already installed. Not sure about prop pitch - how can I tell?

Anyway, I have a suspicion that the fin is not a good idea on my boat - I've got plenty of power on tap to get such a light hull on plane in a hurry - why have the extra drag below the water?

As soon as I get my hands on a GPS, I'll be able to get some reliable speed readings. Then, I'm planning to wait for a day where the lake is glass smooth, and make several runs both with and without the Dol-fin.

Anybody have any suggestions for my "testing"?

BTW, the boat's speedo claims a speed of 38mph with me, one daughter (65 lbs), 10 gals of fuel, and fishing gear.


Clark Roberts posted 02-16-2000 08:18 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Jim, your 15 with the 50 merc probably needs a 14" pitch prop to get an extra 200 rpm.. better hole shot and speeds. The Doel-fin works for me! When properly set up for height and trim the fin will ride right at surface of the water and causes no drag..My suggestion would be, since engine is already drilled for doel-fin, is to try one! There are makes of fins that are "hydro-spoilers" and have a turned-down trailing edge.. these are effective only on stern heavy boats and most outboard set-ups shouldn't use them (Sting Ray is one such brand).. The Doel-Fin is a hydro-foil and actually provides lift aft as boat moves to plane, and it also resists settling and allows the boat to stay on plane at a slower speed. I have had many, many 15's and have had to fiddle with all of them to get the set-up right... the side to side trim is an agrivation... currently I'm running a 79 with a stand/sit center console of my own design and a 40hp 4 cyl Merc (trim/tilt, 14"
pitch Yamaha prop, and a 5" set-back "Bob's Machine MJ-5" jackplate-- motor raised about 3-4 inches) and it runs at 5600 rpm and 35-36
at WOT per GPS. Eric, your 15 with the 70should be running mid-fourties!! Either your engine is not putting out the power, is mounted too low, not trimmed out properly, you have the wrong pitch prop, or something is wrong with the boat's bottom or some combination of the above... If you don't have a tach, you need to get one and find out how much rpm you can get at WOT.. if not turning 5500 you need a lower pitch prop.. if exceeding 5500 you need a higher pitch prop... you need a prop that will allow engine to rev to 5500-5600rpm when lightly loaded and trimmed out... rule of thumb is that 1" of pitch gets you 200 rpm either up or down... Try raising engine a hole at a time until props breaks loose on fast take-off and/or in hard turn, then drop it back down a hole... I have found that satisfaction improves in direct relationship to the "fiddle factor" (the amount of time spent fiddling with engine set-up, seating comfort, grab rails, trim and weight balance.. etc, etc). I'm always fiddling and am never satisfied until everything responds like I want it and the seat and seating position are right... Happy Whalin'... Clark ... Spruce Creek Navy
mark posted 02-16-2000 09:37 PM ET (US)     Profile for mark  Send Email to mark     
Jim, I made my own "whale Tail" for my striper 15 and it has improved performance by allowing me to get on plane faster and throttle back while staying on plane. I was not a believer in those things until my friend put a doel fin on his 13'. Im not sure what RPMs Im running, it,s been a long time since I checked but I seem to have no problem with get up and go with 2 guys and fishing tackle. Im running a 79 merc. 50 with a 15 pitch (I think, its stamped 15P).
I'm also in the process of re designing my side console and moving it to the center. I hope to have it ready near the middle of march. Good luck, Mark

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