Top End Speed

[Crestliner2007]

I am new to this site, so I hope this is the right place to ask my question. I have a one year old Yamaha 15 HP 4-stroke engine on a Crestliner 14' Sportsman tiller boat, also a year old, both bought new in 2006. My engine came with an 11" pitch prop which I quickly swapped with a 9" prop to get some kind of decent speed out of this combination. Went from 14 mph up to 21 mph with the new prop; then, by adjusted my trim, I got it up to 25 mph. I was satisfied at that point, as I know the engine is underpowered for my 35 hp rated boat and equipment. Due to the escalating price of gas, this season I decided to put a Doel fin on the engine to improve the time it takes to get up on plane, which was taking as long as 30 sec. or more! I put the fin on and now it takes only about 3 to 5 secconds to get up on plane and only another 5 to 8 seconds to acheive top speed. My problem is that my new top speed is now down to less than 20 mph once again . I suppose this is due to the added drag of the fin in the water(?). I've tried different trim positions, but the best one is still the one I had last year. My question is, what can I do to improve my top end speed with this rig - if anything? I'd be happy to get it back up to 25 mph if at all possible. Should I try an 8" prop? At about $135. for this prop I hope this not my best option, but I'm willing to try anything at this point. Thanks for any suggestions.

 

[Ray Neudecker]

Re: Top End Speed

The fin helps to get out of the hole but is not allowing the boat to clean up enough for your previous speed. That thing is way overkill on that rig. Start cutting off on it and you may get closer to your goal and not lose the holeshot.

 

[Crestliner2007]

Re: Top End Speed

That makes sense, but how do you go about cutting it? Should I follow the contour of the fin? Or the contour of the original cavitation plate? And how much should I take off at a time? They are not all that expensive, so if I screw it up, replacement is only about $20., but I'd like not to screw it up if possible! Let me know if a picture would help? Thanks for your assistance!

 

[Marc III]

Re: Top End Speed

I did the same thing to a Stingray fin. I took maybe 2" off each side matching the original lines. I cut mine with a dremel and a cut-off wheel and filed the edge flat. The waste material from the cut-off wheel was very hot!! Maybe you could raise your motor a bit too.

 

[Crestliner2007]

Re: Top End Speed

I do not believe that raising my engine would not be a viable option, due to the fact that it is bolted thru the transom. Hate to re-drill the transom on a brand new boat without knowing that it would be beneficial. Not a great course of action to aid resale value! But I like the idea of using a dremel to cut back on the fin. I guess the consensous is that going from a 9" pitch prop to an 8" prop would not be of that great a benefit then in my case, since no one has yet given me any feed-back on that option?

 

[steelespike]

Re: Top End Speed

The best thing is to remove the fin.What wot rpm are you running at?
You can get a tinytach to check,Inexpensive easy to install.
If you don't know your rpm you could ruin your motor.
Adjust the verticle height of the motor.
Raise it about 1/4 inch at a time till just before ventilation becomes a problem.Simply add 1/4 inch sticks on top of the transom.Watch that it continues to pump water.This may give you a little more speed and may help holeshot a little.In extreme set ups this could gain 1 to 3 mph.
If it has regular clamps no need to bolt until final setting.
Once you have it all set if you still need more hole shot then add something like smart tabs.Do a search here.These are very effective even improving no wake speed handling on tail heavy boats. Have excellent tech support.

 

[Crestliner2007]

Re: Top End Speed

steelespike - my motor was originally drilled & bolted in place when I bought it. Adjusting the verticle position now is not a great idea - unless it's my last alternative. Due to preservation of re-sale value. And, you say that "In extreme set ups this could gain 1 to 3 mph" - if you read my initial post, I've lost 5 - 7 mph already by installing the fin! 1 - 3 mph isn't going to do anything to alleviate this situation. A tail wind would do as much without drilling more holes in my transom. I NOW know I should have done the 1/4" incremental raising prior to having the motor drilled in place, but I figured a reputable marina would know how to do this in the first place. This is where I'm at - as posted thus far. HELP!

 

[Crestliner2007]

Re: Top End Speed

BTW, I do not yet have a tach onboard. Never thought I'd really need one with a small rig like mine. If the marina is suggesting going to an 8" pitch prop, they can't be very concerned about over reving(?). I will certainly look into getting one though, if you think this is really necessary.

 

[steelespike]

Re: Top End Speed

steelespike - my motor was originally drilled & bolted in place when I bought it. Adjusting the verticle position now is not a great idea - unless it's my last alternative. Due to preservation of re-sale value. And, you say that "In extreme set ups this could gain 1 to 3 mph" - if you read my initial post, I've lost 5 - 7 mph already by installing the fin! 1 - 3 mph isn't going to do anything to alleviate this situation. A tail wind would do as much without drilling more holes in my transom. I NOW know I should have done the 1/4" incremental raising prior to having the motor drilled in place, but I figured a reputable marina would know how to do this in the first place. This is where I'm at - as posted thus far. HELP!

OK, your sounding a little agruementative.First Thing I Said was remove the fin
bingo 5 to 7 is back.Raise the motor, you may get 1 to 3 mph you also may improve your hole shot a little. Not sure how many holes you have now but if you use the clamps it won't take any holes to raise the motor then after you have the results you can make the decision whether to drill more holes or simply put it back.Most small motors I've seen have 2 bolts It's hard for me to believe 2 more holes will affect the value.By the way proper closing of un used holes has been discussed here many times.
I have had great results with the Doelfin but no question you lose top speed and except for certain specific instances you will use more fuel.
Properly adjusted Smart tabs will out perform the fins on all points.
Depending on your present rpm you can make the decision on a different prop.If you want expert prop advise try the prop forum.The first thing they will ask is Boat type/brand, speed and rpm at wot.also the typical load and activities.
I'm no expert on 4 strokes but I'm guessing at your original 25 mph your pretty close to your max rpm.
Good luck.

 

[Crestliner2007]

Re: Top End Speed

Thanks muchly steelespike! Believe me, I wasn't trying to be arguementative, just that with a new boat, I'm a bit leary about drilling any more new holes. But first things first....the first thing I need to do is take off the fins and get a tach. I will definitely try the 1/4" incremental raising at that point, noting differences in rpm, top end speed and hole shot. This tach...can I get one simple enough for me to attach to the engine? Do they make one that is portable, like a multi-meter type? Thanks again for your help.

 

[steelespike]

Re: Top End Speed

Do a search on google use tiny tach. I'm pretty sure these are very easy to install and can be portable.Not multi meter.These work off plug wires and are specific to your motor type.4 stroke,2 stroke,1, 2, 3, 4 cyl. etc.
You might look here at iboats don't know if they carry tiny tach.
Any improvement on raising the motor will depend on the present setup.
A prop change can help combat ventilation.
Be sure to watch that the motor continues to pump water.
Once you get a final setup you can decide on a prop.You may be able to go to a stainless prop if you want to try for more speed and combat any ventilation problems.Not sure what is available for smaller motors.But you can go to the prop forum with your setup
and specs and they can help.I'll be interested to here the results.

 

[Crestliner2007]

Re: Top End Speed

Thanks again steelespike. I'm off to iboats! Now, forgive me, but I should take OFF the Doel Fins prior to these tests, correct? Will the holes in the cavitation plate affect my results? (4 x 1/4" holes)

 

[steelespike]

Re: Top End Speed

Yes remove the fin.Try it without filling the holes.If ventilation seems to be a problem early in tests try plugging the holes.It has been discussed here but I don't remember the details.It was a simple procedure.As I recall tape on one side and some sort of filler on the other.A quick search may find a solution.As you raise the motor the plate may not even be in the water.