Would this work?

[altura]

Boat has a 20hp diesel, suppose to go 6knots, 30ft long, 10ft wide (a bit too wide id think). Remove the keel, upgrade engine to 40hp.

Shouldnt it travel at 10+ knots?

[Trevlyns]

Whoa!

There are so many variables here, that I’ll only venture a start!

First, the situation as presented (if I read it correctly) “converts” a sail boat into a motor launch. Totally different concept and hull shapes here… The original keel would be used to maintain balance and reduce the heeling forces from the sails. Remove it and (retaining the rounded hull sections) you have a very wobbly machine likely to tip over on the first turn.

Next, rudder area is greatly reduced – how will she steer?

And why double the power to gain a measly 4 knots. Get a multi-hull sailboat and get these speeds – plus – without an engine at all!

[altura]

Basicaly i had two ideas, attached photo is my first idea.

Was thinking since the macgregor26 power/trailer/sailer powers ok without turning over why would this sailboat, i know its got a more rounded hull though.

I can get the boat for basicaly nothing, wanted to be able to trailer, 28ft long/9.5 beam.

[Trevlyns]

Yeah, the Macgregor 26 powers ok because it is designed as a sailboat. It has a keel to keep it stable. Remember, the keel also contains the ballast (on average 40% of total displacement) mounted low down [think of the pendulum effect] Remove it and you also remove stability.

In my humble opinion, even if you can get it for next to nothing, it will not work as a motorboat without the keel. Why not keep her as a sail boat?

Best!

[altura]

Cant be trailered or launched.

[alan white]

Originally Posted by altura

Cant be trailered or launched.

Not without a permit.

[PAR]

The Mac 26 is a powerboat that happens to have sails. It also requires huge amounts of power for its class of sailboat.

Not knowing what type of sailboat you're looking to bastardize, it's difficult to speculate, but most modern sailboats can be driven to around 8 knots without worry. They may do so under sail already in plane mode.

Sailboats designed with the intention to plane off on some points of sail, will have high initial stability, which is a result of the boat's shape. The appendage comes to play when the boat is sailing with some heel on and for directional stability (as well as lateral). In a modern boat the ballast weight will roughly be somewhere between 25 and 45% of the displacement of the boat.

So, in a nutshell, you'll remove 25 to 45% of the boat's weight (appendage and ballast) from the very bottom of her, then add a quite large superstructure atop her and hope for the best. Have I got this right?

It can be done and has been done, but wouldn't be a recommendation for someone not well versed in the principles and dynamics of yacht design.

With her beam, you'll need special over size trailer permits. Most states charge around 50 - 100 bucks a year for over size load permits under 10' wide.

Frankly, except for a small portion of gunnels, you'll be replacing the vast majority of the boat as you've drawn. Why bother, just build what you really desire, rather then a hacked up want-a-be powerboat from a sow's ear sailboat.

[altura]

Here is some pictures of the hull. The boat supposedly weighs 8000 lbs with 3500+ lbs ballast. 28ft long 9.5 beam.

If i removed all the sailing gear etc, cut the keel much shorter, maybe 90% off, cut the rudder down too it would fit my trailer. I dont want to sail, i want to got 8mph+ and stable, would it tip over easy? or would it be similar to a stable 28ft trawler?

[DanishBagger]

Why not hinge the rudder on the rear instead? That way you could take it off when trailering, and you could make it balanced too.

90% sounds a bit much. My guess would be 60-70 percent. But I think you need a naval architect for that sort of thing – and all the numbers and figures - to estimate that sort of thing. Otherwise you could try a little by little. And be ready to scrap the whole idea, of course, and torch the boat.

[altura]

I never thought about hinging the rudder, thats a good idea.

I would have thought someone would have removed a keel from an old sailboat and used it as a powerboat.

But maybe a boat like this just tips right over without the keel even when the complete sailing equipment is removed.

[alan white]

Removing the majority of the keel is going to narrow the waterline beam as the boat rises due to having shed 3500 lbs. You could sink the boat back by taking the lead and putting it in the bilge (which isn't a real deep space).
Not a bad idea except that the weight has no structural value, so it is very much dead weight, needed only because the boat's shape needs it there to stay at the design waterline beam AND length. It is pushing a rounded bottom into the water.
Still, it's an efficient displacement speed boat that should go 7 knots with about 20 hp.
You would need a skeg to protect the rudder, and the skeg would want to be continuous, curving under the prop. The rudder could be a bit smaller, and more horizontal.
I don't see why you need 40 hp. That hull will eat the extra 20 hp and spit out a half-knot for your troubles, at twice the fuel consumption.
You can do 7 mph at 2/3 throttle, I'm sure, with 20 hp. Without the keel, you should get an eight mile an hour top speed easily.
It's not my business to say the idea has merit. If it appeals to you to do it, why not?

[DanishBagger]

Imagine removing the keel (the suggested 90 percent), and then loading it all up with golf cart batteries and an electric engine! 1,7 ton of batteries! That would propably make it cruise on batteries for quite a while (at a time).
But with batteries and little electric engine, it wouldn't be dead weight!

(Can you tell I'm sold on hybrid propulsion?)

[Guest625101138]

A slight variation on what has so far developed it to cut the keel down to a few inches below the prop. Fit a skeg in front of the rudder and run a connecting length of solid steel between the bottom of the keel and rudder skeg. Steel bar about 4" wide and 1" deep. Really solid and capable of bearing the boat weight without yielding.

This would provide physical protection for the prop and rudder as well a good resting beam for the boat on the trailer.

I think you will find the boat will actually be more stable than most motor boats without any additional internal ballast. It will not hurt to float a bit higher so long as you get the full waterline when under power. I would think about 7.5kts with 10HP. You lose some windage without the mast and the weight is less.

The metacentric height actually reduces as you load the boat to the waterline. Unloaded it will be around 2m above the keel and reduce to around 1.6m as it gets down in the water.

The motion might be somewhat sharp with the rounded chine, no sail or mast and ballasted keel but I expect it would be seaworthy.

Rick W.

[retired racer]

Nooooooooo! Don't ruin a decent boat that way. Take the thing for cheap and fix it up then sell it and buy your motor boat with what you get from it. Or if you don't think you can fix it then take out the lead, burn the boat and sell the lead for scrap. Sell some hardware and rigging too.

And btw, the beam is probably 8'.