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Author Topic:   15 Classic with F60: engine height? prop?
PMUCCIOLO posted 04-20-2003 10:05 PM ET (US)   Profile for PMUCCIOLO  
My brother and I took the 15 out for its first spin today. The F60 is incredibly quiet, smooth, and smoke-free. It is truly superb.

For the second hour of operation, per the owner's manual, the engine is to be slowly accelerated every ten minutes to wide-open throttle and then brought back to 3,000 RPM's.

At every speed over 3,500 RPM's, the boat begins to porpoise. No trim change or movement of its two passengers could eliminate this problem. The engine, I believe, is mounted at the third hole on the bracket. If that is too high, would the situation I've described result?

The engine only turns 4,900 RPM's wide open. It is supposed to turn between 5,000 and 6,000 RPM's. Is this purely a function of the prop's being too large, or can the engine height be the culprit here as well?

Any suggestions to reduce the trial-and-error method would be greatly appreciated.


Perry posted 04-20-2003 10:27 PM ET (US)     Profile for Perry  Send Email to Perry     
What size prop is on the motor right now. It seems you could go down at least 2" in pitch to gain 400 rpm. Where is the anti-ventilation plate in relation to the bottom of the hull at the transom? With this added input you will get more accurate answers to your questions.
PMUCCIOLO posted 04-20-2003 10:38 PM ET (US)     Profile for PMUCCIOLO    

The prop is a 10.5 X 15 polished SS three-bladed OEM Yamaha model. (They offer a 13 inch version of the same style.) Should the engine be near the top of its maximum RPM range (i.e., 6,000) when lightly loaded?

As best I can tell, the distance between the cavitation plate and the bottom of the boat appears to be about one-and-a-half inches.

Thanks, in advance, for any assistance and advice!


Perry posted 04-21-2003 01:02 AM ET (US)     Profile for Perry  Send Email to Perry     
Paul, you should gain 400 rpm by going down to a 13 pitch prop but that still may not be enough. As a general rule, you should prop your boat so you reach the upper end of the rpm range with a light load. When you say that the distance between the anti-ventilation plate and the bottom of the hull is 1 1/2", do you mean the the plate is above or bellow the hull? The distance sounds good if it is above the hull.
PMUCCIOLO posted 04-21-2003 08:05 AM ET (US)     Profile for PMUCCIOLO    
The cavitation plate is about one-and-a-half inches above the bottom of the boat.
PMUCCIOLO posted 04-21-2003 09:22 AM ET (US)     Profile for PMUCCIOLO    
Apparently, the motor is mounted too high. It is supposed to be mounted within 3/4 inch of the bottom of the boat. The prop is also going to be dropped to 13".


Bigshot posted 04-21-2003 10:00 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
That aint too high. My 70 you could see half the power trim motor it was so high and she ran fine. Something aint right though.
Wild Turkey posted 04-21-2003 10:40 AM ET (US)     Profile for Wild Turkey  Send Email to Wild Turkey     
I second Bigshot, I don't think the engine is mounted too high. My setup is 1-1/2" above the of the boat (cavitation plate). I do not have any problems with this setup at all (Montauk, 90HP, setback plate).

You should set it up to hit 6000 rpm under light load conditions. You mention two passengers in your first post, I would suggest maybe just the captain for light load conditions.

I can't imagine needing a 13 pitch prop for your setup. Something is strange. I have heard that some outboard motors "loosen up" after break-in. I do not know if that is the case with 4-stoke motors though.

PMUCCIOLO posted 04-21-2003 10:53 AM ET (US)     Profile for PMUCCIOLO    
This needs further investigation...I'm going to try the boat, per your suggestions, alone today. A full report is to follow.

Thank you for your input!


jstachowiak posted 04-21-2003 12:26 PM ET (US)     Profile for jstachowiak  Send Email to jstachowiak     
The only times I could get my 15GLS w/40HP 2S Johnson to porpoise (and boy does it ever porpoise, scarry) was when the motor was trimmed to far out. To get it to settle down I would trim it in until it stopped.
jameso posted 04-21-2003 12:52 PM ET (US)     Profile for jameso  Send Email to jameso     
Engine height should not cause the porposing. You are probably trimmed too far back, Do you have T&T, if so trim the engine all the way down then raise trim slowly until perfomance is at max with no porposing. If manual trim do the same moving one hole until max performan is achieved. GPS is good for this. My 15's have never porpoised unless really trimmed too far out.
The engine height sounds OK. If you have a good hole shot and the thing does not 'blow out or cavitate on turns you are OK.
Check the trim. Also how much static trim (weight) is in aft of boat? Static trim should put water line, about an inch and half below the engine scuppers. This is sitting at dock light load no passengers. Hope I have not confused this.
Bigshot posted 04-21-2003 01:27 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Although she is PTnT, there is a manual trim adjustment bar(just like on a manual tilt engine) that sets how LOW it can trim under, move that bar to the closest hole you can in towards the transom.
PMUCCIOLO posted 04-21-2003 07:23 PM ET (US)     Profile for PMUCCIOLO    
The post on the engine bracket which limits the engine's negative trim is in the hole closest to the transom. Even with the engine trimmed in all the way, it still hops a bit. Clearly, something isn't right here. The boat is going back to the dealer.

I reviewed some photos of my family's other 15's with my brother. The boats all had OMC'c, ranging from 40HP to 70HP. As best we can tell, the (anti)cavitation plates were all level with the bottom of the boat or just above it. Also, it appears as though the part of the bracket that hooks over the transom was very close to the transom cap itself. My boat has about a two inch gap there. The Yamaha bracket could be responsible for this, as its design may vary in appeance and relative position from that of the OMC when similarly installed.

Please keep up with the input, as I'm trying to get this resolved.


lhg posted 04-21-2003 09:55 PM ET (US)     Profile for lhg    
Paul - With power trim, there should not be any pin through the transom bracket. Remove it, and you can trim in more. I'm speaking from Mercury experience, but I would assume this would be the condition with Yamaha also, as Nick mentioned.

Engine height sounds right to me, but you should be using a performance prop, like the Mercury Trophy Sport 4-blade, if it will adapt. A Stiletto prop would also work.

Have you communicated with Clark Roberts? He seems to be the expert on 15 performance. I believe he also contributed an article in the reference section on 15's.

PMUCCIOLO posted 04-22-2003 06:25 AM ET (US)     Profile for PMUCCIOLO    

I'll drop a note to Clark. Thanks for the advice.


Clark Roberts posted 04-22-2003 08:50 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Paul, I don't think I can add to the excellent posts above... this one is a mystery. I, also, don't think it's engine height but who knows! Sounds like a case for experimentation, that is unless the hull has a "rocker" (factory defect): a rocker is a curve downward of the keel (should be straight) and a boat with a rocker cannot be trimmed and will bounce and porpoise no matter what! A "hook" is a curve upward of the keel which will cause bow to dig in...etc..etc...etc...If your hull has a rocker then factory should fix or replace. If no rocker then experiment with height, prop and distribution of weight (more forward)... good luck... clark.. Spruce Creek Navy
David Jenkins posted 04-22-2003 11:50 AM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
Clark, I would never have guessed that you had experience with rockers and hookers :) My 1977 Sport 15 has sat on a trailer so long that it has an indentation of about 1" where the rear-most roller meets the boat. Its diameter is about the size of a frizbee, and the center of this indentation is located about 6" from the stern of the boat. Would that make my boat a rocker or just a rocket?
PMUCCIOLO posted 04-22-2003 06:40 PM ET (US)     Profile for PMUCCIOLO    

Thanks for the suggestion. I shall check the bottom of the boat. Are the curvatures to which you're referring parallel to the boat's bow-to-stern axis or its side-to-side axis?


Clark Roberts posted 04-22-2003 07:32 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
David, the indentations could be enough to cause handling problems but I doubt it.. usually amidships rockers/hookers are more noticable.. hookers can make you rock.. heh, heh... PM parallel to fore/aft... actually the keel-line! Look for the keel to sag about amid-ships. A long straight edge or maybe a string can be used... if boat can be lifted off trailer it would be easier... Porta Marine/Products in Edgewater (428-7417) may be able to determine for you or just get Whaler to take a look. If you can't stop the porpoising by moving weight forward then something is wrong with hull (assuming that engine is mounted correctly... like proper height and plumb)... Clark.. Spruce Creek Navy
PMUCCIOLO posted 04-22-2003 10:09 PM ET (US)     Profile for PMUCCIOLO    

Thank you for the clarification and the referral. I shall investigate...hopefully THE HULL is not the problem.


elaelap posted 04-23-2003 12:22 AM ET (US)     Profile for elaelap  Send Email to elaelap     
Very, very strange...I get 5900 rpm/30 mph GPS WOT with my 243 lb F50, 13" pitch aluminum prop, anti-cavitation plate exactly level with the "keel" of my heavier (I assume) Katama 16/17, battery under the console, 17 gals of gas under the bench seat, 40+ lbs of salmon weights in the bow locker, 12 lb anchor, line & chain rode in box in front of the console, and 200 lb me. I get almost exactly the same performance with my 165 lb son aboard -- I sit him on the box in front of the console (or he sits me there when he's drivin'). Loolee3's got smooth fresh bottom paint, just to round out the factors. Very, very strange...


PMUCCIOLO posted 04-23-2003 06:40 PM ET (US)     Profile for PMUCCIOLO    

The prop was changed and negative trim plates (or something of the sort) were added. Apparently, the motor hit 6,000 RPM's without difficulty.

Reportedly, it could be trimmed out at high speed without porpoising. That is, in my opinion, what separates the 15 and 17 classic hulls the most--the ability to run the boat with the last foot of the hull contacting the water!

Of course, this remains to be seen. My brother and I are going to test the boat tomorrow. I shall keep everyone abreast of the difference in performance.

Once again, thanks for all of the support and advice.


PMUCCIOLO posted 04-25-2003 01:49 AM ET (US)     Profile for PMUCCIOLO    
The propeller and trim angles were the culprits. A smaller prop and a bit more negative trim at lower speeds did the trick. The boat planes easily with minimal bow rise and it is FAST!

After it's broken in all the way, I'll obtain some objective numbers with a hand-held GPS.

Thanks again for everyone's assistance. I'm glad to report a happy ending!


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