add-on keel or skeg?

[n0ukf]

Dad just got an old lapstrake style fiberglass 19' Larson with a short shaft v4 Johnson 75 hp (yes, short, it's a 15" transom). I noticed during the test run before purchase that it really leans before biting into a turn and easily ventilates the prop. Would there be any decent way of adding a keel or skeg or whatever to this boat to make it bite into a turn earler, or do we just live with the heavily leaning turns? Dad just mentioned a boat he had over 30 years ago that had a flat bottom, and I think he said something about skegs to help against sliding sideways in a turn.

 

[Spidybot]

Re: add-on keel or skeg?

If the hull is deep V, it will lean. The deeper V designs lean a lot, but this is how it works (and why it does not slide in turns). A flat hull will slide and a keel or lists can help.<br /><br />The lean comes from the prop's thrust when the motor turns on the transom. This is also the reason why a side wind will make it lean towards the wind (you have to steer slightly contra to stay on course).<br /><br />The ventilating issue is a different ballgame. Air is drawn to to prop and this causes ventilating if the prop design cannot cope with this air/water mix.<br /><br />To prevent ventilating you should check motor mount height on the transom (x-dimension), antiventilation plate height in relation to the keel line, trim angle and, ofcourse the state of the prop.<br /><br />Having the motor a little high on the transom helps speed (as it reduces drag) but needs a prop that will hold it's grip when surfacing.<br /><br />What rpm at wot do you reach?

 

[n0ukf]

Re: add-on keel or skeg?

With the motor mount resting on the top edge of the transom, how can the 15" motor be lowered without cutting into the glass of the 15" transom? Maybe we can play with the trim a little when it's back in the water. Are there special props available for the old '63 75 hp Johnny?<br />(My 14' Glastron has a 2x2 or less on top of the 20" transom to raise the motor, but I'll have to look at that arrangement again after rebuilding the transom.) <br /><br />I have no idea what the WOT RPM is, no tach. We were only out on a test drive before paying for it. 75 hp feels a little light for this 19', but it does get it on plane with 3 people in it. And it's a LOT smoother on a 1' LOTW chop than his '73 15' tri-hull.

 

[Spidybot]

Re: add-on keel or skeg?

Sure the V is smoother than a tri-hull on chops. Cuts like a knife in butter compeared to the tri-hull that pounds away on top of it. The tri-hull has other great benefits, but chops a re hard on them.<br /><br />You should measure the transom - from the top edge of the transom, where the motor hangs, down to the very bottom of the transom (keel). This should match the leg length.<br /><br />If this measure is 20", the 15" motor simply is too short. Rather than cutting out and lowering the ol' 75'er it would be more better to get the power you want in a motor designed for the transom height.<br /><br />At least 75-80% of the max rated hp for the boat is a common advice. I'd guess it's rated for 150 so 115hp would be a possible goal (even about the largest you can get as 4-cyl, thinking about mpg).<br /><br />Can't say what props would be available for the old motor. Problem though, that hi performance props (surfacing) hardly fits the setup as they normally come in high pitch and you'd want low pitch on a '19 foot+75hp' combination.<br /><br />I know that this most likely is not the answer you're looking for, but start off with measuring the transom.

 

[n0ukf]

Re: add-on keel or skeg?

^^^^ It's a 15" transom, so the motor supposedly rides low enough.

 

[Spidybot]

Re: add-on keel or skeg?

OK - the idea was to check if the manufacturer actually made it 15" or only approx.<br /><br />If the antiventilation plate is at or close to the keel line, the trim angle is the next suspect. As a starting point the antiv. plate should be parallel to the surface while planing.<br /><br />I think finding props for this one could be hard - maybe second hand is better.<br /><br />Rpm is important in deciding what prop to look for. The old prop has some numbers, telling it's pitch. Have you checked what it is?