Lobster boat conversion into a picnic

Hy

I'm considering a project to convert a lobster boat into a fancy pic nic boat.

The skeg looks huge and the hydrodynamics worst.

Does anyone have any experience in cutting the skeg and leaving the bare shaft supported by a P strut?

Thanks in advance

 

Welcome to the forum.

You can remove the skeg and run just a shaft/strut setup, but as with everything, there's some trade off's. Without knowing what you boat is (make, model and year) it's hard to suggest much, but that skeg doesn't look so huge and it's hydrodynamics will be sorely missed if you run just the shaft. It's removal will also render no protection for the prop or the rudder, which also loses it's lower support.

A partial skeg, maybe one of more diminutive scale can offer prop and rudder protection and support the lower end of the rudder. Lastly, judging by the little I can see of this hull, the lose of tracking and lateral area will probably be quite noticeable in a beam or following sea. You'll spend a lot of time "chasing" the helm back and forth, because of this, which is annoying to say the least.

In short these relatively slack bilge turn, round bottom hull forms need a skeg, if you want good, predictable manners underway.

 

There will be a very minor improvement is reduced drag , probably not enough to justify the loss of handling at low speeds.

Running aground would be less damage with the stock setup.

If speed is the game , the engine choice will have far more to do with performance.

At 1/2 Mile per gallon great speeds can be had from a lobster hull.

Usually a gas engine is used for speed as 300-400HP diesels are heavy and expensive , an old Corvette LT1 can be replaced overnight thru proper hatch design , as required.

It might be cheaper and faster to purchase an out of favor boat with huge engines , and chainsaw the motoryacht deck house and install a lobster boat looking deck house.

All the nice stuff would be there already.

 

Hi boatrestoration,
can you tell us a bit more about your boat? For example: hull shape, max speed, installed power and a photo of the hull, if possible.
Cheers

 

If you cut off the skeg, you will notice a marked increase in rolling.

 

skeg removal

Thanks to all the replies

The basic consideration is fuel economy as the blunt fore and trailing edges of the skeg look like dragging a lot.

The engine is a 160 HP diesel boat data is

LOA: 34 ft 0 in
Beam: 10 ft 4 in
Maximum Draft: 3 ft 6 in
Dry Weight: 11500 lbs
Engine(s) Total Power: 160 HP
22" X 18" four-blade bronze prop

I'm attaching a photo where the hull can be seen. It look pretty round.

Maybe sharpening the fore and trailing skeg edges will be best,to keep the directional and antirolling stability intact. Anyway boat wil be used on protected waters.

My main concern is the big gap the the trailing edge leaves before the propeller. I guess that will compromise the efficiency of the propeller due the turbulent water feeding the prop.

 

I would advice you to keep the skeg for both directional stability and as anti-rolling device, as Gonzo has pointed out. Huls with round bilge tnd to have a more tender rolling characteristics than a hard-chine ones, which is not a desireable characteristics for a picnic boat. The skeg, if sufficiently long, can act as an effective roll-damping device there, and if it was my boat I would keep it.

From the picture of your boat and the installed power, it looks like a displacement or a moderately fast-displacement boat. At these speeds the shape of the forward part of the skeg is not really important, but the trailing edge can make a difference. A cut-off skeg, like the one you have, creates turbulence which increases hydrodynamic resistance, degrades propeller's efficiency and can trigger the onset of cavitation at high prop loadings.

So, rather than removing the skeg or changing it's shape along the leading edge (forward part), the best cost/performance-improvement ratio could be obtained by a simple streamlining of the rear part of the skeg, the one which is facing the propeller blades. This picture will explain pretty well what I mean:

(taken from http://features.boats.com/boat-content/2010/07/johns-bay-boat-company-bucks-the-tide/)

The picture in your first post shows how it should not be done on fast-displacement boats (if your boat belongs to that category).

Cheers

 

Boatrestoration, your assumptions about the gap aren't correct. The gap looks pretty much average, not overly large, though the skeg, as Slavi mentions, could stand some streamlining, to clean up the flow to the wheel. You can round the leading edge of the skeg if desired, though you'll have a limited efficiency improvement. If you round the leading edge, don't sharpen it, but just carve a 2:1 ellipse into it. For example, if the skeg is 8" wide, draw a 8"x16" ellipse and of course, use half of this shape (attached).

Taper the trailing edge as far forward as you can, starting at 50% aft of the leading edge if possible. In most cases, you can't get get this much taper without threatening the shaft, but get as much as you can. Lastly, don't make this taper "hollow", like that used on a chisel tip, but make the shapes uniformly convex.

 

When a motor boat ,without a keel , runs aground it pushed the P bracket thru the bottom, the prop cuts a hole in the bottom...then it sinks as you call your insurance agent and the rescue authorities

Dont even think of removing the keel.

 

Damn, thats a lot of damage from running over a bicycle!
(sorry, couldn't help it, my very observant 10 yr old nephew pointed it out)

 

Seems to be a few threads on the forum with this subject, it seems remarkable how many of these keels end abruptly with no attempt to streamline.

 

skeg edges

Thanks for the tip. I'm more concerned about the trailing edge. I have seen a good solution in another forum just in another lobster boat.

I'm attaching the photo solution

Joe

Miami

 

Good example

Dear michael

Thanks for the tip. However that hull on the pic is crap. It hasn't been reinforced properly to support the strut. Here in Miami most groundings are soft, just sand and the 3'6" draft of the project boat makes groundings more difficult.

Joe

 

Good solution

Thanks this what i was looking for. I have published some photos based on this solution modifying the trailing edge on a similar lobster boat.

 

That boat sank in the harbour.

It was standing off the fuel dock wating for space. The skipper wasnt paying attention..the boat drifted slowly until it was overtop of the submerged part of the rock jetty..the skipper gave it some gas to motor clear of the jetty..crunch....thump thump..end of story.

Id rate the construction layup of the boat to be typical...

The lesson is that non keel protected props need a waterproof bulkhead that encloses strut and rudder