Changing a full Keel on my boat

It certainly is a planing hull, but I'm sure some would consider it a semi-planing or semi-displacement hull, your boat has somewhat more engine hp than most of them it seems, and would benefit more from the type of alterations you are considering, than the lesser powered ones. The NMPG figures you quoted were very poor, way too bad to tolerate. I note that a portion of the keel is moulded grp, so that should likely have to stay, but it seems quite feasible to me to design a much-abbreviated keel set-up as you have indicated. Effect on stability, I would not be too worried about, directional stability characteristics would change, but should still be OK. Frankly it beggars belief how that keel ever got into production, especially for a 250 hp set-up, it is far from ideal. But that is apparently how they designed it, then again it may have been with much smaller hp in mind.

 

I have been noticing them around (owning one of them makes you do this) and have seen only 1 other with a 6 cylinder cummins , 3 or maybe 4 with 3208 cats, one with a 6 cylinder volvo and stern drive, around 8 with ford lees diesels , but i think mine has the highest hp rating i have seen.
It would be easier if it was a more popular boat, or maybe someone else had been down this path before, but I've got what I've got (at the moment a boat that doesn't travel in either mode very well, and burns a lot of fuel doing it).
As far as drivetrain goes, I've rebuilt the engine, replaced the engine coupling, resealed the trans , relaid the engine beds, remounted the engine onto the new beds, renewed the shaft and coupling and replaced the cutlass bearing. There isn't too much i haven't done on it, but I've got the jitters about removing the keel. That is why i am hoping to get some good feedback on my project.

 

Is she a bit broachy ? If it is, you may need to be careful about what you do with it. What is the set-up with the sterndrive boats, any keel to be seen ? Hard to imagine you could do anything to that boat that would make it worse, fuel-wise.

 

I actually haven't seen the underside of the stern-drive one, and am scouring the boat-sales ads to find it. From memory it only had 200 hp but cruised at 18 kts and ran to 24.

I don't recall it being broachy, but I haven't really used it much either.

The speed is one thing, but I cant see the sense in owning the boat if I cant afford to run it, and currently I cant.

 

To think of my repairs from another perspective, if i were to simply cut away the current keel and properly fair the trailing edge will there be any advantage in a keel that's thinner ? The wetted surface area would be similar for a plate steel keel of the same profile , and as mentioned before the flat wide keel bottom would contribute some lift , so is there any advantage to changing the keel to a plate steel style keel or am i better off carving timber?

 

i remember 1 boat that was 32 x 8 ft which had a small keel about 8 inchs deep, it planed at 14 knots with a 4236 perkins. the owner decided he wanted it to track better in a following sea so he added a keel very similar to yours. now it won't go over 8 knots. not much help for you but does show how big the difference is. just remember that if you cut your keel down you may weaken the boat if it was designed for the large keel.

 

The strength issue was the reason I was considering a steel keel - to maintain longitudinal strength and handling without the frontal area of the current keel. I cant see how the vessel could perform worse at all from what I'm proposing, but I'm not a naval architect.

I've now got hold of a 4 blade prop thats suitable, I just need to get it depitched and the center bored for my shaft.

I've also been reading up on adding on some spray chines/rails to lessen the wetness of the boat. currently the only time the boat is reasonably dry is flat out, once you've given the boat enough power to get the forward chine clear of the water. while fun to drive like this, not many could afford it.

 

I think you have a well burdened yacht that could tolerate the complete removal of that skeg (what you're calling a keel). Clearly she'll do well in semi plane mode and has some modest potential at full plane , particularly if the drag of that monolithic monstrosity was removed. You could keep the shaft and rudder, or go out drive, which would be my recommendation to really "clean" her up.

A spray rail at the chine could greatly improve the dryness of the boat and isn't a hard addition. In this vain, getting some performance improvement with strakes will require a fair bit of design work, to be effective with lift. She looks to have a chine flat forward to knock down spray, but this might need to be carried further aft and possibly extended a bit, to be more effective, particularity if needing additional lift.

The skeg could be part of the structure, but I doubt it. I think it's just an old school way of keeping the shaft from getting beat up and it's easy to build, particularly as poorly as that one appears.

I'm not trying to put your boat down or anything, it's just I see something different from the previous observations in this thread. I see a moderate forefoot, with some buoyancy and a warped bottom with sufficient deadrise at the transom (I'll guess 8 - 10 degrees) to handle deep water, with stability and comfort.

An out drive wouldn't need this skeg at all, though a considerably smaller one might be installed, to help with steering and handling properties, in certain conditions.

I think you can get more speed, better fuel efficiency, climb up on plane faster and much crisper maneuvering with an out drive upgrade. If you stick with the shaft, just going with a naked shaft and some struts would help tremendously, though a small skeg would be wise. It needs only be wide enough to contain the shaft and offer some protection. A 6" appendage is way over the top in this regard.

 

The chine flat is only about 1 1/2" wide and not at all effective. I have been on the bow and watched the spray as the boat accelerates and the water doesn't really even deflect away from it, it just sort of forms a tubey bubble where it is . I was thinking of around 6" spray chines.

The skeg (keel), I like the idea of keeping something there for the protection and the rudder support. Outdrive isn't really an option due to the unload facility at the wharf where I berth.

I guess the end result I'm chasing isn't too far from what I have, except for dragging around that railway sleeper .

If I were to go with a steel skeg, I was thinking of around 12mm (1/2") vertical member with 12mm x 150mm (6") against the hull to bolt the skeg to the hull. Would this be strong enough?? What would you suggest for the lower end of the skeg (to touch the bottom when I stuff up or when I put it on the slipway cradle)

 

this is a pretty standard style of semi disp hull on the west coast of aus. some do use the skeg as you call it to stiffen the hull. all i am saying is to check before he chops.

 

I have as a matter of decency, tried with the original builder to get the drawings for the stresses and stability but apparently they were thrown out years ago. If you look at the drawings, the moulded part of the skeg (keel) will be kept (its where the wooden part bolts on) and the wooden part removed and replaced with steel. (at least this is the plan at the moment) In general, the hull is way overbuilt. They were originally built as trawlers for the rivers and estuarys of the mid north coast of NSW. They do have a straight run aft from a fairly full bow though and may pound a bit in the rough if pushed, but then the throttle isn't a light switch.

There is a resemblence to the Randalls they use for the crays though and they are a tuff boat.

 

If you are sure of the relation propeller/engine/ tranny /speed right now, meaning they work well together, I will not change anything.
It will cost you a lot and you will take a great chance to have bad result if you start surgery.
You says you are navigating in a bad area, meaning I suppose wind, current, waves, shallows, and for that you current keel design, althought not elegant is a the best solution.
We do this type of keel here in Maine, it works well for the area.
Probably derating the engine, trying a different prop, not trying hard on the speed, will be a cheaper and more secure solution.
If I was over there, I will be glad to navigate with you, but without having trying the boat, what I said is an opinion, just that.
I personally like this kind of keel, protecting the propeller and the rudder. But that's me

 

Don't overlook the option of just selling it and getting a better one.

 

I want to keep the keel, I'm not really interested in getting rid of it. I'm thinking now the question is , is there any real advantage between the narrow profile of a plate steel keel over a thick but properly faired timber keel/skeg ? Would the plate steel end up lighter? The cost isn't really a huge consideration mainly because i can see it being recovered fairly quickly if the gallons/mile figures get a bit better. The main consideration is getting the right focus on where to pick up any efficiencies so my time on the slip is used correctly.

 

There a huge amount of drag associated with that skeg and most of it isn't necessary. I'm fairly familiar with this type of craft, as they're common in most places with active fishing fleets.

A small skeg will help to some degree in low speed handling and beam seas, but other then shaft and wheel protection, the rest of that monstrosity is just eddie making weight and drag. You could have one just as strong from a fraction of it's weight, that offer as much protection too. Any time you can do this, you'll gain efficiency, which pays for itself.

Also, making some spray knockers forward and/or making the chine flat more functional, in terms of lift and spray direction, would also be a good plan.