Hull extension

B

Billy

New member
I went to the local boat show and noticed that something new being added to many of this year's boats is a hull extension. Probably not all that new, but the first time I noticed it. The addition I'm talking about is not a full width extension of the hull and allows for trim tabs, but a sort of swim step over the top of it does go full width. I figured this was the manufacturers method of offsetting the additional weight of the heavier 4 stroke engines and it might be the answer to mine too. I went to a 90 hp Yamaha summer before last and still wanted to add larger fuel tanks but was concerned over all of the weight I was adding to the stern. Currently I take fuel drums which eat up deck space. So I designed a similar "stinger" for The Beerthirty and it is undergoing a hull extension as we speak.
Because I will no longer need the splashwell for motors, I have enclosed it to make a fishwell, which means one less ice chest on deck. Also, by extending the hull, I have in effect, moved the fuel tanks forward which should also help with balance.
One of the concerns was not to add too much more displacement to the stern and upset the original performance characteristics so I went with 18" in length to about 32" in width which V's up to meet the width of the old transom. After subtracting for material weight, I figure I'll net about 200 lbs more displacement and lots more clear deck space. I'll try to post pictures tomorrow of the process.
Bill
 
Billy,

Let me know when you plan to do the first sea trial. I would love to follow and take pictures to see how the boat planes at various speeds.
 
Bill,
From your description, it's hard to tell what you are talking about. What the difference between what you are mentioning and a bracket for an outboard?

Roger on the SeaDNA
 
Tom, I'll get ahold of you when the time comes and maybe we can get out at the same time. It would be great to have some pictures and a second set of eyes.
Rodger, maybe the pictures I hope to post tomorrow will help. I would guess that a bracket is just a mounting platform for the motor? This is maybe better described by trying to imagine the whole boat stretched 18" with a section notched out on each side for the trim tabs. It will be part of the hull and provide additional flotation. Sorry, I guess my description wasn't very clear.

Bill
 
Wayyyyyy cool! I'm very interested in this one. The transom is the only thing I really don't like about the C-dory. You'll have lots more useable space getting rid of that splashwell. Our old 21' Fiberform had a beefy aluminum transom bracket that a previous owner had made up when they removed the inboard and replaced it with an outboard...and I loved it. Used it as swim step, too.

Curious why you decided on wood instead of aluminum?
 
416rigby,
The plywood you see will be all glassed, inside and out. Heavier certainly, but how the fiberglass guy wanted to do it. I could have done without much of the box if I had gone with a full width extension but there was a major cost difference. I talked first to a guy about doing it in aluminum and he was a little hesitant about aluminum on fiberglass. Not that its a problem by itself but if you chose to paint the bottom you have to be careful on which paint you use. Copper based paint reacts with the aluminum apparently. You are supposed to use zinc based I guess. The big reason though is he didn't have time to do it this year and I still would have wanted the fishwell glassed in.
Bill
 
Billy said...
One of the concerns was not to add too much more displacement to the stern and upset the original performance characteristics so I went with 18" in length to about 32" in width which V's up to meet the width of the old transom. After subtracting for material weight, I figure I'll net about 200 lbs more displacement
Click to expand...

One concern and one suggestion...connection of new transom joints on rear addition where the motor will hand to the side pieces...epoxy and lots of screws...I assume...then glassed both sides?

Consider filling the inner cavity with pour in foam for more buoyancy?

I know you didn't ask ..... I just couldn't help myself.
Mike
 
Billy-

Interesting and gutsy project!

If it works, you 're going to be a genius! If it doesn't you'll be something less than a genius! The generally prevailing theory on such modifications is to not mess with the original design concept, as you know. You won't really know for sure how it works until you're in big water, especially in following seas. Here's wishing you the best!!!

Did you consider going with a ready-made transom motor bracket like the Armstrong? ( I don't know if they would be suitable at all from dimension or other considerations.)

Extra gas tanks can be added as saddle tanks, keeping the added weight more amidships. (See Red Fox's album or website.) Did you consider this?

How do you think the placement of the motor 18" or so aft will affect the overall balance of the boat in terms of bouyancy, sea worthiness, and especially dynamically in powering and steering the boat?

What motor did you have before the Yamaha 90 4-s, which weighs 369 lbs? What did it weigh? Could you feel and tell the difference after switching? In what way?

Looks like you're having a boat shop do the construction for you. Who are they? Have they done this type of addition before?

Sorrry to ask so many questions, but with a project as out of the ordinary as this one, you're going to get the "3rd Degree" on everything sooner or later anyway (!).

Keep up the flow of photos! You've really got our curiosity up and making us think out of the box!

Joe.
 
Papillon, Yes to all of the above attachment suggestions but I think no to the foam. I have thought a lot about it and may end up doing at least a partial fill of that cavity yet. Here's my thinking. First, the added buoyancy would only occur if I swamp it and that area fills with water, otherwise the displacement is there either way but I like that idea. Second, I need access to the heads of the motor mount bolts and the fishwell drain. The new fishwell drains using the old splashwell drain and a length of hose to a new through hull fitting on the starboard side of the new addition. Again, your idea could still be incorporated by only partially filling that space with foam.
Thanks for your ideas,
Bill
 
Seawolf Joe,
I appreciate all of the friendly questions. I had lots of questions too when I started thinking about this and I haven't answered all of them yet. One question you had regarding following seas... I don't really know how it's going to act but I did think a lot about it.
The thought process behind this modification was to restore the original design performance carrying the extra weight I do, not change it or think I could improve it.
For example, I went over and saw Greg's saddle tanks and they are even nicer in person than the photos show but Greg has a Classic Angler and I have a Cruiser. There are differences in the cockpits of each, and some of the other things I've added that make that modification impractical for me.
In keeping with restoration vs new design, I toyed with the idea of going full width but this added more displacement than I needed to offset the weight I had added. I tried to shoot for close to the amount I had added, but definately no more. So I was shooting for about 300 lbs added displacement tops. The boat came with a 70 hp Johnson and two 18 gallon fuel tanks. The Yamaha I think weighed about 100 lbs more as I recall and double the fuel equals out to a total of about 330 lbs. The extension is beefier than I expected so I might net about 200 lbs more displacement from that. Still 130 lbs over original design concept. Moving the motors aft 18" has the reletive effect of moving the fuel 18" forward even though they will be in the same space under the splashwell. That's not exact but you get the idea. This should have some of the effect of Greg's saddle tanks.
I think the sum of the changes moves the performance back toward original design characteristics as opposed to anything new.
Unanswered questions:
In a following sea, are the cavities on each side of the extension (trim tab space) going to act like a cup for the sea to grab? Don't know. A possible solution if it does is to fill that area in or round it out. Can't get anyone to sit still to make tabs useful anyway.
Does this change turning characteristics? Don't know that either. I don't think so, but if it does I would think that it would only show in tight turns at high speed. Something I really don't do, but would be interesting to know.
Something I didn't think of was is there going to be enough room between the new transom and the new fishwell to fully tilt the motor? Maybe 18" is enough, don't know. I didn't think about that.
The absolute worst case on this project? I injure myself with the chainsaw cutting this idea off but you'll all hear about the good, the bad and the ugly.

Bill
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I finally figured out how to reduce the size of the other photos I had and get them into my album, so there are some new ones of this project in there.
Starcrafttom, disregard the PM if you want, these are the ones I was going to send to you.

Bill
 
Bill,
Quite an ambitious project. What are you going to do about your trailer? You may need a little extra length to get support for the hull extension. Otherwise looks like a lot of weight cantilevered out past the bunks. Also where are you going to mount the transducer for your depth finder? Will it still fit in the old location? This is a very interesting modification and I look forward to more progress reports. Looks great so far.
 
These new pix really do make it a lot clearer. And we are confident that this extension will be sturdy enough to hang outboards on so they won't pull or twist the whole sucker off?
 
Wow! This stirred up more interest than I thought. Well, no matter how this turns out there seems to be entertainment value in it. I'll try to answer all of the questions I can remember.
Ken, I'm certainly hanging out off the end of my trailer but I always felt that I was over taxing my single axle anyway. There is a second axle and parts waiting in the garage for when I get her home. The transducer placement was relegated to the "I'll figure that out when the time comes" bin. I really didn't think about it until I started removing things for the extension.
Pat, That's the hope, keep your fingers crossed.
Dan, I look at this addition like this is easier and cheaper than a bigger boat. I think it will give me all that I want without starting from scratch. Displacement. I used the scum line (OK I don't keep it as clean as I should) for one of the measurements and got 18X32X12. With 250 lbs (minus added on weigh), the reletive shift in fuel tanks and the greater planing surface it seemed worth the effort. No drain plug yet but this is still a work in progress. Access hatch. I had to add a fairly large (10X20) watertight hatch to the top of the swim step over the extension to accomodate attachment to the hull and interior glassing. Although much bigger than I had originally planned, it also allows for access to engine mounting bolts.
In answer to someone who asked if this was like an engine bracket and to another who suggested an Armstrong bracket, I have since looked those things up and I would say yes there are a lot of similarities. This is just my adaptation of someone else's idea. Like I said, I saw it at the boat show.
Bill
 
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