Motor height

[Cranky18]

I have a 14' aluminium boat with a 15hp motor. I am having problems getting the boat to plane unless I move weight (people) forward. The ventilation plate was about 3" below the bottom of the boat. I put a shim in to bring it up even with the bottom. After doing that I saw no improvement and have severe ventilation everytime I hit a wave. Any suggestions? Here are some photos.<br />

 

[Dhadley]

Re: Motor height

A couple of things -- with a keel that comes that far back you have to consider that the lowest part of the hull. So when your AV plate is even with the transom edge, its still above the true bottom. <br /><br />That being said, its hard to tell but it doesnt look like the prop has much bite or cup. What kind of rpms are you turning? If the motor is overpropped it will really struggle to get on plane.<br /><br />Then, what is that mounted on the bottom to the left of the keel?<br /><br />I sure hope that board is more soild than it looks.

 

[Cranky18]

Re: Motor height

Thanks DHADLEY. The board pivots and it was very windy yesterday so it seems the board is up against one of the "chines". I have a 12 pitch prop but the guy who repitched it pitched it to a 10. The thing next to the keel is where the drain is. I am thinking I need to remove the shim and buy a new 10" pitch prop. <br />Also, the exhaust hub is missing from this prop.

 

[CATransplant]

Re: Motor height

How many people are you talking about moving around to plane the boat? You may well just be too darned heavy to plane the boat with a 15hp motor.<br /><br />First off, I think the motor's too high. If it's ventilating all the time, then it's too darned high.<br /><br />Further, that extension, as Dhadley said, looks way flimsy. That, and the height it is, is likely to send your motor to the briny deep one day. I'd put a good safety chain on it. Looks like you just have it lag-bolted into the transom. If that's the case, I'd do something else to raise the motor...but somewhat less high.

 

[Cranky18]

Re: Motor height

The extension was only to see how raising the motor affected the boat It is secured inside with a metal plate (and yes, 4 8" lag bolts).I plan on replacing the transom anyway. There is also a cable attatched from the motor to a metal bar inside the boat. It is just one person I am having to move from the middle seat to the front.

 

[CATransplant]

Re: Motor height

OK, that's probably strong enough. How wide is this boat? I'm still thinking you're at the very minumum hp to plane this boat anyhow with two people on board. How does it do with just you?

 

[CATransplant]

Re: Motor height

Plus I see that it's a tiller motor. Depending on your own weight (a delicate question, I know) you could be down at the stern a little too much for easy planing, even solo, with that little motor.

 

[Cranky18]

Re: Motor height

The boat is about 55" wide. It used to plane before the prop was "fixed" with just my son and myself sitting in the back 2 seats. I have not tried with just me yet. From what I have read, the 15 horse should be enough to get at least 2 of us on plane. (it used to)All of this started when I had the old prop repaired. All three blades were badly bent on the edges. I suppose it it is possible that someone before had overpropped it and tried to compensate by bending the blades. According to Mercury information a 12" pitch is a the upper limit anyway. Basically I am trying to eliminate the motor height as the problem. Appreciate the help!

 

[nevd]

Re: Motor height

I had the same motor on a very similar boat and make the following comments.<br /><br />12 pitch is only of value for 1 person and very well finished boat as the motor will not rev out. <br /><br />You are better to be underpitched than over as the motor will reliably over rev. I used mine for approx 60 hrs at approx 7500 RPM on a 9 pitch prop and speed was almost identical (with 2 people) on 9, 10.5 and 12 pitch, but with 12 pitch, it was very slow getting on the plane and then would only do 5200 RPM.<br /><br />I damaged one prop and lost the diffuser ring and the prop was useless with bad ventilation until diffuser ring was replaced.<br /><br />A repaired prop is very suspect as the pitch may not be the same on each blade and this will also decrease performance.<br /><br />Generally, this motor needs 12 pitch for 1 person, 10.5 for 2 people and 9 for 3 people with the motor as high as it can be run consistent with the ventilation being controllable.<br /><br />Care is needed with high motor mounting to ensure the motor gets cooling water which incidentally comes from under the cavitation plate and not the leg like all larger motors.<br /><br />Hope this is of value.

 

[Cranky18]

Re: Motor height

Thanks for the info nevd. I am trying to find a new 9 pitch here locally. I am going to put the motor back in its original position and try it like that.

 

[Cranky18]

Re: Motor height

nevd, worked exactly as you said. I could probably go with a 10 pitch but it gets on plane quickly with all three of us aboard. It goes as fast as my wife and son can stand.