Keel Modification

As stated previously, common practice is minimum 10% of diameter. Adding a blade is a common way to get this clearance the easy way, without modifying anything.

I put a lot of faith in prop guys, its what they do. I do like to get several opinions from several prop guys, and the one that is willing to work with you should be the one that you listen to most closely.

Rather than any of the expensive or labor intensive things you're thinking about, maybe ask around about the pro's and con's of an extra blade. It's hard to beat diameter for power, but boats are always a compromise.

Another thing I've seen done by some in my group with a similar set up is to fair the back of the keel above and below the shaft box to a taper rather than squared off. The thought is the squared off back of the keel creates turbulence tight in front of the prop and fairing it to a taper lessens this turbulence. I did it on my Mainship 34 but at the same time as a re-power so I don't know if it contributed or not, but some of the guys in my group claim good results and it does make sense.

Steve

 

I hear you, I have run things past a commercial fish boat builder and the prop guy. Problem is I'm repowering and powering up, and the prop pocket that is existing will at max allow a 21" wheel, and a 21" wheel of any combination of blades and pitches won't absorb enough power, it has to be a larger wheel and increasing any distance means a decent amount of work, but certainly not out of the question.

 

New propulsion system

You appear to be on the right course and there are lots of questions and answers here. My 2 bits worth.
1. What QSM are you using: 5.9 or the 6.7.. or ??
2. What is the predicted speed of the boat. Appendage drag may/may not be an issue.
3. a 24" might work, depending on 1 & 2.
4. My experience on a lot of boats like this is 26" to 28" dia. might be more efficient.
5. A 'straight' keel/skeg makes life much easier. Here in the PNW we also have rocks & deadheads.
6. I assume you have checked if the the shaft dia. is okay (or you have a new one, correctly re-sized)
7. If your speed increases, you should check the rudder stock dia.-especially if you fit a spade rudder. (ie: no shoe)

This is a considerable investment and might be worth hiring a knowlegable person to help you

 

Nicely summed up

 

Have you thought about making a tunnel? I made one for a guy who was converting a troller into a gillnetter. He wanted to push the prop forward and out of the way of the net. We managed to do this and gain a little clearance at the same time. This was a 42’ full keel displacement hull. After the tunnel he got speeds of twelve knots smooth as butter. That was a big improvement from before. The big fat keel right in front of the prop blocked a lot of water. He was wasting his time if he tried to go over eight knots. He couldn’t use half of the big giant engine he put in.

Food for thought and a pic. Go easy it was my first tunnel.

 

While the OP did a great job of initially presenting his case there were some bits of critical information left out as you have noted. Hopefully he'll fill in the missing data in his response to your post which by the way is worth a lot more then two bits. All excellent points!

MM

 

Another option and one well worth considering as it addresses many of the issues the OP is concerned with.

MM

BTW...nice job!

 

The engine is a QSM 6.7 425, ZF 280IV gear, 2.25 ratio, the builder of the boat says they would do 20kts light (mine weighed 18,500 with 300G fuel and the old Detroit 6V53) with high HP, a good cruise is predicted at 16kts with a fuel sipping speed of 8.5kts. I was trying to keep the shaft angle as flat as possible, but possibly that won't have much detrimental impact and if shaft angle can be increased I believe a 26 can be in the realm of possible. The prop guy said 24 x 26 would work so I know that is the bottom of the benchmark. Indeed a keel that is flat from front to back is preferred, and yes I'm well aware of the rocks in the Puget Sound, though I'm happy to say I have only brushed up on them with dive gear and not running gear.

For the project the whole nine yards gets replaced-engine, gear, shaft (upsizes to 2”), rudder, etc....

I have thought of the tunnel, but haven't done much research on one and haven't really seen more than handful of pictures. I suppose it could be an option. Any disadvantages to one? Other than the obvious fiberglassing investment. I saw on the ol interweb where a guy once took a similar boat, 32’ of same make but a twin screw, and repowered and took off all the underwater gear, it was two keels with captured rudders, and it was a twin tunnel hull to get shallow draft for Bristol Bay. The guy ended up with hanging rudders and a couple shaft struts and no keels The only comment I remember reading was that he would be better off with out the tunnels because they broke up the running surface at the back of the boat, but maybe one single in the middle won’t really make much issue since it will leave the majority of the flat running surface of the stern intact??? I’m certainly open to a tunnel, especially if it could alleviate having a drop in my keel.

And lastly, that comment about knowledgeable help? That’s what you guys are, knowledgeable help…..! Don't worry, I haven't even broke out so much as a butter knife on this project, just some pictures and CAD sketches and brainstorming, still a long way off from surgery. Thanks for all the input thus far, very much appreciated.

 

Gillnetter,
With this new information in hand I would suggest modifing your plans to include a 26" diameter wheel rather then the 24". The larger prop will better compliment your 425 hp & 2.25 gear reduction combination. Here JSL's comment #4 is especially appropriate now and I completely agree.

Your prop guy's recommendation of a 24"x26" over square wheel is further proof that you should consider a larger wheel. In my experience a prop whose pitch is 80% of the diameter gives the best overall performance. I've never been a big fan of over square props for various reasons and in your case I wouldn't recommend starting out with one.

MM

 

Thank you MM, perhaps a 5 blade 26 with a shallow tunnel? 10% tip clearance enough for the tunnel application? Can a 5 blade get away with maybe 8%? I have 21" between my stringers and a quick sketch shows that if I were to fit a tunnel in there I would get about 3.5-4" of height based on a 31.2" diameter tunnel radius, 2.6" of tip clearance on 26" prop. This height would still require a slight drop in the shoe, but it could be accomplished with a shoe and some simple fiberglass fairing into the drop shoe, not a bunch of glassing to extend the whole keel. I'll work on a mockup to post.

Afteryou- do you have any more pictures of your tunnel fabrication?

 

Unfortunately I was called in to this job a little late. The owner had already cut the keel back and glassed in the back of the keel 6-10” wide. They had also cut out a hole over where the prop was and glassed in a small bubble, for lack of better words. Fortunately he had to go fishing on another boat and called me to finish. When I came to see the project I told him it would never work. But alas I was told to leave the back of the keel alone and that it would work fine. The bubble I flat refused to deal with and was told to do what I saw fit. So I got my saw and started hacking. My point in telling you all this is to explain why this was done in two stages. It did not work for $#%^ and the next year he had me fix the fwd section like I wanted to in the first place. I will add that he was very humble when I came back to right the wrongs.

Some more pics.

 

Makes me itch just looking at them....!

 

That's what keeps me in business

 

prop sizing

I did a cursory look at the prop size based on what info was given and making some assumptions.
Power= 419bhp@3000 w/ red'n=2.25:1

(1) Speed(max) = 22.5kn
(2)Prop: D= 26", P=26", DAR=0.61, blade = 4, Blade area(total;min)= 314 sq.in. Tip clearance (ideal) = 5.2", Min= 3.9"

If the prop was in a tunnel* the size etc. would be the same but you might get away with less clearance ( 2.6").
*Tunnels I have used are about 0.3D [recessed( 26 x .3= 8")]. You could go more but steering might suffer.
For this speed an entrance angle of the tunnel would be about 12 deg. max.

With the possible exception of clearance, this sort of agrees with other posts. But, we all know about GiGo: the results are only as good as the input.

 

You need to be a bit careful.

What the engine will deliver and what the prop can take and be suited for the engine...is different from what the boat shall do, owing to its dimensions, shape and weight.