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Author Topic:   Another prop question II
Lars Simonsen posted 06-23-2003 09:35 PM ET (US)   Profile for Lars Simonsen   Send Email to Lars Simonsen  
I have had several opportunities now to take my 1998 Dauntless (135 Optimax) out several times since raising the motor (up two holes to the third hole) and putting on a new Stiletto 19 inch pitch prop. The positives are higher top speed, and the boat seems to sit higher on the water when on plane. The negative at this point is that the boat seems "stern heavy"; it's a little slower getting on plane, and when coming off plane, I get a wake that comes about halfway up the upper cowling on my motor. I also get some water in the splashwell.

Yesterday, I put an aluminum hydrofoil on the motor. I took it out this evening, and that made for an improvement, but it's hard to tell how much of an improvement since it was a lighter load (just me in the boat). The hydrofoil did lift the stern out of the water somewhat, and as a result, I got on plane more quickly, and I can more easily reach redline on the motor. I also seemed to pick up some top speed (if my speedo was telling me the truth, I got up to 50 mph). But the stern still drops pretty deep when coming off plane, and I get water higher onto the motor than I'm comfortable with.

Would there be any benefit to dropping the motor one hole? Or would that have the opposite of the desire effect?

The hydrofoil I put on is an old "Whale Tail"; it's basically just an aluminum plate. Do the Stingrays and Doel Fins have some "engineered" shape to them that give them more lift?


lhg posted 06-23-2003 09:58 PM ET (US)     Profile for lhg    
I have never known engine mounting height to effect the following stern wave. That is usually a function of hull design. You may just have to drop off plane a little less quickly. The Stiletto prop does indeed give you additional hull lift, which is desireable. I have heard that the Dauntless series of hulls have a tendency to "squat" in the stern on acceleration and coming off plane.

Besides hull design, acceleration is usually a function of prop design, number of blades (more is faster) and venting, if any. That's why Mercury props come with the 3 sizes of vent plugs. The larger the vent holes, the quicker the acceleration. But 50 MPH is pretty respectable top speed out of that rig. Those 135 Opti's sure do run. If you need more acceleration, and slightly less top speed, you should be running a vented 4 or 5 blade prop. Mercury's 4 bladed Offshore, 4 bladed Revolution or High Five come to mind.

I do not believe in anti-ventilation plate mounted "trim tabs/stern lifters", so can't comment on that installation. Seems like too much drag to me during acceleration. I figure if they were needed, the engine designers would have designed the anti-vent plates that way. I couldn't bear to drill those holes in the engines!

JohnJ80 posted 06-23-2003 10:10 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Hi Lars,

It sounds like all that you have done a lot to improve the performance of your boat.

What size Dauntless do you have?

When you get hit by the following wake, is that going from full throttle to dead stop? When you say the 'upper cowling' do you mean the fiberglass cover that you lift off the engine to look at it or repair it? If that is the case, I'm unsure of why it doesn't come right into the boat over the transom in a very big way. If I am understanding you correctly, half way up on this would be over the transom. I think its normal to get some in the transom well if you chop it big time (depending on how much).

On the fins:

Usually the fins also slow the 'squat' of the boat down when it comes off a plane. They shouldn't increase it, I would think.

I used to have a stingray fin on my Johnson 70HP 2 stroke. There was a measure of improvement over the performance of the boat without one.

I just repowered and I put a Evinrude 75HP DFI/FICHT on the back (a lot heavier - about 80 lbs) and a Grand Island Turbo Lift fin (benefit, you don't have to drill the av plate). There was a larger improvement with that combination that with teh Stingray. I think too, that the Evinrude was a more difficult application for a fin because of the weight. So, bottom line, I think there is significant differences between fins.

I moved my motor up all the way to the top from one hole up from full down. This also improved the ride dramatically, especially in chop/boat wakes.

I just added a stiletto SS prop (13 1/4x15) tonight - changing up from an aluminum one. I got much better attitude on the boat to the water, the ride improved yet again and the trim range was dramatically better (like I could trim all the way up in certain very flat water conditions). The boat felt like it was riding much higher (stern lift?) in the stern than before.

With all these changes, I think the performance of my boat with this motor, new prop, the hole positioning and the Turbo Lift is superior to what it was before with the old motor.

Lars Simonsen posted 06-23-2003 11:07 PM ET (US)     Profile for Lars Simonsen  Send Email to Lars Simonsen     
Believe it or not, I got that much of a wake up the cover of my motor, yet had very little come into the splash well. It happens primarily when coming off plane pretty quickly. I'll just have to develop the habit of coming off plane more slowly. I think this is probably caused by the "transom notch" or tunnel in the hull.

lhg, I agree with you that it seems that the motor manufacturers would incorporate the hydrofoils into their design "if they worked." But at the same time, it makes a lot of sense that a hydrofoil does provide a lot of stern lift. I guess not all boats need the extra stern lift, so it doesn't make sense to put them on all outboards. I cringed a bit drilling holes in the motor, but it was all in the name of science.

The hydrofoil I put on does slow down the squat of the boat when coming off plane. I think I just need to run the boat some more to see how much improvement it has made. Also, I'll have to develop some new driving habits.

John, I have an 18' Dauntless. I'll have to take a look at the turbo lift. It'll be a while before I can allow myself to spend more money on the boat, though! It'll have to wait . . .

The Optimax motor definitely puts out some power. But I'm still suffering some post traumatic stress from the untimely demise of the first powerhead less than a year ago. With each hour of operation, I'm gaining a bit more confidence in the reliability of the new remanufactured powerhead I had installed. It is sort of a thrill to hit 50 mph. The boat actually felt like it was floating on air!

Sal DiMercurio posted 06-23-2003 11:35 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Lars, you were riding on air, thats why you hit 50 mph.
With the boat riding much higher on top of the water, that means less hull in the water, thus less drag, thus more speed.
I really think the water is coming up your stern because your running much faster & when you chop the throttle to fast, the boat has to break back through the water & comes to a faster stop once it does.
Compared to running lower in the water & running slower, your already broke through the surface & maybe not desturbing the water as much?
Really not certain about that but makes me think why....
Pretty tough to beat a Stiletto especially for that price.
It's amazing how just a change of prop design can make the whole boat ride much higher on the water.
Hey, 50 mph with a 135 isn't to shabby.
bsmotril posted 06-24-2003 11:17 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
I guess that to make your performance runs, you are trimming the motor up quite a bit. Before you come down off plane, first trim the motor all the way down/in. This will slow the boat quite a bit, then reduce throttle to idle. Doing this will help a lot in keeping the bow down during the transition which in turn will reduce stern squat and lessen the wave that floods the motor.
JohnJ80 posted 06-24-2003 01:09 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Just a comment on the fins -

Motor manufacturers have little to no idea of what boat you will put their motor on. Different lift is needed for different configurations. There is no way that the motor manufacturer can anticipate what you will do with the boat, whether or not you will need lift and how much or whether you intend to push a displacement hull or plane with it. They don't know what you average sea state is that you operate in, they don't know how you ultimately intend to use the motor - is it speed? Is it a good ride? Is it a workboat? They no nothing about the load you intend to carry or what you will do with it.

So, they err on the side of less underwater drag and provide enough AV plate to stop ventilation of the prop and provide a good enough environment for the propeller to deliver its horsepower through the shaft.

In point of fact, a better source for whether you need a fin or not would be the boat manufacturer. They know what the hydrodymamic properties of the hull are coupled with the range of motors they spec. They allow for any motor provided it meets the Horsepower constraints, weight ( in some cases, not all) and transom height. After that, the load and the rest is up to you.

So, I would submit, that the least knowledgeable of all in that chain as to whether you need or don't need hydrodynamic lift at the motor would be the motor manufacturer. Better information would come from the boat manufacturer and then better from you once you figure out what you need.

This is exactly what I discovered. The mfg and the prop guys don't know (although one prop guy told me that any boat that planes can benefit from a fin if the motor is at the proper height). The boat guys (BW tech support) were more helpful, but ultimately, it came down to me, as the owner, experimenting and figuring it out.

lhg posted 06-24-2003 01:29 PM ET (US)     Profile for lhg    
It seems to me that if a boat needs stern lift under various conditions, put trim tabs on the boat, not the engine, the least efficient, most unattractive, and highest drag option. Several Whalers come with trim tabs from the factory.

Several years ago "Powerboats Reports" did an extensive study on various brands of engine mounted "fins", and found them all to be harmful to performance, not helpful, and generally a waste of money. Note, I'm just the messenger here. I have never used the things, but have noted that some here have posted results of 3 MPH loss of top end.

Shiney new outboards do not deserve to be disfigured in this manner!

Lars Simonsen posted 06-25-2003 09:58 PM ET (US)     Profile for Lars Simonsen  Send Email to Lars Simonsen     
I can't say for sure, but based on this one ride with the hydrofoil on, I do think I picked up some top speed (I hadn't hit 50 with this set up before the hydrofoil). The boat also seems to get on plane a little easier. Trimming the motor down as I come off plane has a noticeable effect on the height of the stern, and does reduce the "drop into the hole" effect. However, I'll have to do some more boating to know for sure whether it really offers any significant improvement. In the meantime, I'll practice coming off plane more slowly (which probably will be a more comfortable ride for passengers, any way, and using the trim to keep the stern up.

I truly is a thrill to feel the boat airborne!

lhg; the fact that my motor is basically 5 years old (not so shiney and not so new) certainly made it easier for me to disgigure it by bolting a plate of aluminum on it. Perhaps my impression that it improved things is just wishful thinking since I had to maim my anti-cavitation plate to mount it. But I'm leaving in on there for now! If the only effect the hydrofoil has is to make me able to keep the stern from squatting so much when I come off plane, that's worth it (since at 50 mph, the hydrofoil certainly can't be to much of a hindrance to top speed).

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