Keel Modification

Brand new here, I have an ol 36' gillnetter that I'm giving a heart transplant, out with the screamin DD and in with a Cummins QSB. The hull is full keel semi displacement hard chine, captured rudder, 21 x 21" prop which is pretty small for the boat. I would like to get a 24" under it which of course means I need more clearance, so I need to scoot the prop aft to get more clearance. In order to move the prop aft I have drummed up a couple options, all of which entail different degrees of fiberglass time investment and all result in, I assume, different degrees of drag. They also amount to concessions on prop protection and to whether or not the boat can sit on bottom, presently it can with full keel and captured rudder. I attached, at least I think I did, the scenarios I drummed up, pdf format.

#1- extend the whole keel aft and deeper, keep the captured rudder, fair in the shaft exit, quite a bit of work, but not out of the question, and allows boat to still sit on bottom.

#2- keep the captured rudder, but extend the keel in a more open design less drag I assume, add a strut mounted top and bottom, still lots of work, but less than #1, boat sitting on bottom may be marginal.

#3- cut the keel back and add nothing, add a top mounted strut, make the rudder hanging, fair in remaining keel, least amount of work, most prop exposure to damage and assumed to be least amount of drag.

#4 Same as #3, but add enough depth to keel to avoid issues if the boat sits on the bottom.

I'm up in WA in the San Juan's, so plenty of mud bottoms to be found. I guess the decision might boil down to how much less estimated drag results between #1 and #3, which I am estimating to be the most and the least amount of work, and also providing the most and the least prop protection. If #3 has a large estimated drag benefit I might be inclined to run the risk of prop protection. Any input is greatly appreciated, this is the only boat I have owned so no great experience with hulls and drag and different designs.

 

Hi Ol

Welcome to the forum.

Before you even consider doing what you want to do, changing from a 21” prop to a 24” is not a simple as taking one off and replacing it like a shoe.

A prop’s size is dictated by the rpm, the power and the speed of water flow into the prop. Have you checked the implications of changing your prop against you existing engine?..since I assume you’re not changing the engine and gear box too?..or are you...is the DD meaning Detroit Diesel? Not 100% clear.

What is the objective of changing the prop?

 

I realize repercussions of prop size and pitch. This is a repower, not just a prop change; new engine, new gear, new shaft, new prop, only thing iold after this will be my poor old wallet. My prop guy already calculated my needs, that is why I'm aiming at a 24", but I'm trying to feel out the brains of the marine world to see thoughts on the underwater mod's with respect to gains or losses due to design. I see Naval Architect on your thread, exactly the brain power I need..... Thanks

 

It sounds like you've answered your own questions. In the sense that you say there is plenty of mud bottoms about. So, do you wish to sit on the mud, or anticipate grounding on the mud frequently?

I wouldn't worry about the differences in drag, it'll be minor. The key issues are your practical aspirations for the boat.

 

So you think drag differences between my ideas is pretty negligible. I do take the boat to shallows and hop out on beaches often so the ability to sit near or slightly on bottom is a nice feature, always sand or mud. As well, changing zincs is a lot easier at low tide with the boat fully dry. I suppose if drag is not a consideration then it's just how much effort to expend to be able to park shallow. Thanks much

 

Ol

Yup..i wouldn't worry too much about the slight difference in drag.
Being able to utilise the mud flats is far more important, I assume, always good in an emergency too.

What is also does too..is stiffen up the shaft line. Since the s/s bracket option would require some transverse stiffness to prevent the shaft from vibrating/wobbling, especially if you dink off a blade, or something gets attached to the blade. The eccentric forces can be high. So the "traditional" capture rudder is nice and stiff. So long as you have done a few sums to prove it too

 

Hi Gillnetter,
Of all of the options you've shown the one most commonly employed by myself and majority of fishermen in my area is a less radical version of #1.

Going from a 21" to a 24" doesn't require some of the major surgery you're proposing. if this were my job I'd 1) Remove the shaft tube 2) Remove 3" or 4" of glass from below the tubes current location or drill a new hole 3) Reposition the shaft tube for adequate prop clearance and 4) Reglass.

You'll have to rework the skeg and rudder a bit but everything else can and should stay just about the same. There are always variations on a theme but that's it essentually.

You should be aware that dropped skegs do have a tendancy to hang up on corklines, so you want that transition to be smooth. Also adding a beaver tail and extra supports adjacent to the rudder post insures against prop, rudder or hull damage should things go wrong the next time you're out digging clams.

MM

 

Not quite clear on your project but if you are modifing the keel why have a P bracket ?

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MM, I'm trying to keep the shaft angle as flat as possible, though maybe it won't matter as much? I do anticipte about a 1.25 deg angle change so I would be both increasing shaft angle and scooting the prop back. Thought as you say, I could simply go heavy on the angle increase and not scoot the prop back? What is a recommended max shaft angle in relation to the hull? I would need to increase shaft angle by about 3.25-3.5 degrees to fit a 24" prop in existing prop pocket and not move the rudder. Also this boat is retired, she got out of fishing in 99, just used for recreational purposes.

 

I'm pretty sure that if you started a thread on shaft angles alone it might go on for hundreds of posts with dissenting points of view and opinions ad infinitum. The simple fact is it will make almost no preceptable difference, just as your proposed keel modifications will do little to decrease drag. Yeah you want to keep the shaft angle to some reasonable figure, but this will have more to do with the engine mounts and insuring adequate engine lubrication then anything else.

I think every boat in our fleet has undergone a major repowering over the years with the attendant question of "how do I fit a bigger prop under there?". The easiest and most common solution to the problem has been to increase the shaft angle and modify the skeg and rudder assembly to accomodate the new position. There may be some theoretical advantages that can be postulated for each of your various solutions, but a modified version of #1 offers you the most protection, the simplest build and the lowest cost. Not to mention that it's also the one best suited for "shallow parking" as you so eloquently put it.

Since the boat has been retired from gillnetting you have some latitude when it comes to repositioning the rudder if needs be. Normally I'd insist on keeping that right where it is, but in your case moving it back a few inches to allow more room for the larger prop might be in order. Since you plan on replacing the shaft then adding a couple of inches there might offer some additional relief in terms of hull clearance. I like to keep that clearance at or above 10% of prop diameter or 2.5" to 3" in this instance...less then that and you'll start to notice some vibration in the hull.

All in all I like your plan I'm just saying that you can accomplish your task without cutting away major parts of the keel or going to some of the extremes shown in your various drawings. Keep as much of your keel intact as you can and make some minor adjustments to your layout and you'll have a win win situation.

MM

 

Good advice, I've worked with the angle a bit and it looks like while I will still have to move the rudder, the prop will only need to move back a little and the angle increase will be about 2.75deg, so probably not enough to lose sleep over.

 

I looked pretty hard and see no mention of "number of blades." Did I miss it somewhere?

Steve

 

Plan is to start with four, and end with four. We'll see how it all holds up.....

 

Has your prop guy talked about 5 blades?

Steve

 

He did mention the options as basically 4 or 5 blade, but said to get proper 24" wheel clearance if I could. I think I heard a 5 blade deals with low clearance cavitation better??? My present 21" has 1.25" clearance and makes a racket....