john allmand engine mount reinforcement rebuild and stringers.

Discussion in 'Powerboats' started by dillrepair, Jun 19, 2016.

  1. dillrepair
    Joined: Jun 2016
    Posts: 2
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    Location: lake superior

    dillrepair New Member

    hi. not really sure where to start. the boat is a 1981 john allmand offshore fisherman. not sure what engines originally were equipped, wasn't able to find that much about it online. it is possible this boat never had twins to begin with, but i know it was offered with that option.

    please see these pictures in this imgur album (no spam pic hosting site) for reference to my project:

    i have some fiberglass knowledge and have read and re-read the west system guide to stringer and bulkhead repair and reviewed other internet sources but i have some structural questions and i want to proceed in the best way i can without breaking the bank on this project.

    there is clearly a lot of fiberglass work to be done all over the place including deck work, transom work, stringer work, and strut/rudder mount areas.. i have no concerns with most of it however the engine mount towers seen in the picture are a concern for me. two or three are delaminated at the base and flexing but all will clearly have to be removed and redone and reinforced but i want to know what everyone thinks is the best way to do so. they need more lateral and longitudinal support but i don't want to create any "hard spots" (if thats something i should even be worried about i'm not sure) or make things any worse.

    the boat is still fairly rigid in its current state (can lift most of a whole side off the trailer from a corner with just a few inch lift on a jack) and is on an a-frame trailer. the aft end of the a-frame trailer was modified poorly to make it wider so from the shaft logs back the trailer poorly supports the boat. i've read to support the boat at the stern corners and as far forward on the keel as possible to keep the hull convex when doing stringer repair is this correct?

    i was already planning on adding 2 larger bulkheads for deck support and to create separate bilge spaces 1 under the helm where someone cut out the main cross member and another under the intact cross member. should i add small low bulkheads laterally connecting the mounts to the hull and each other in the engine compartment? can/should i use marine ply to add diagonal/longitudinal stiffness? how much "gap" should bulkheads or added reinforcement have at the hull and should i use some kind of foam base to allow minimal flex in the structure?

    the engines on this project boat run smoothly despite their age so i'm going to be sticking with them. they are inline 6 chevy 292 blocks with velvet drives. each one is almost as heavy as a v8. when i got the boat they were wildly out of alignment and the logs are worn but serviceable. surprisingly both shafts are straight within 3 thousandths. someone had done a very poor job repowering and mounting as you may see in the pictures. you can see how high they are in the cockpit i don't think i have a choice about that unless i move the locations of rudders struts and logs completely (don't really want to that)

    i'd greatly appreciate any and all constructive advice.

    thanks in advance,
    Last edited: Jun 19, 2016
  2. spencyg
    Joined: Aug 2016
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    Location: Northeast

    spencyg New Member

    I don't have a whole lot to add to your posted questions, but I did just buy a '79 Allmand 26 OSF and am glad to see somebody else is out there working on one. Mine is a single engine (Crusader 350, 300hp) running thru a 1:1 Velvet Drive and swinging a ridiculously small 13"x17" propeller. I had the opportunity to sea trial the boat before purchase and though the boat rides like a dream, the small propeller and bad transmission ratio led to a cruise speed of just 13kts at 3k RPM and 20kts WOT ~ 5300 RPM. It was cavitating from around 4600RPM on up which is of no surprise. The largest downfall to the design in my opinion is the prop to hull clearance. With twins as you're working with, It is likely possible to run more efficiently with smaller wheels, but I'm curious how much clearance you have with twins as opposed to a single down the CL of the hull. I'm actively working on a plan over the winter to change the transmission ratio and move the whole underwater gear setup aft and down to allow clearance for an 18" wheel.

    As for your structural concerns, I agree that your engine mounts look...precarious. At the very least I'd look into the bulkheads as you suggest. If it were me, I'd be curious about the viability of fabricated stainless bed risers that could be bolted to the stringers without the large blocks being involved at all. Given how much more efficient a larger diameter propeller is with heavier planing boats, you might seriously consider actually lowering the engine and getting the shaft assembly further down into the water. I know this isn't something you really want to consider, but the long term usability and efficiency of the boat would be significantly improved.

    Best of luck!

  3. dillrepair
    Joined: Jun 2016
    Posts: 2
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    Location: lake superior

    dillrepair New Member

    i think the props on my twins are the same size as your single. there wasn't all that much clearance. maybe a few inches. i'm almost certain the boat wasn't really meant to have those big straight 6 motors in it... i'm assuming it had twin 4 cyl. originally. i think you're right though, with the amount of glasswork i have to do at this point it might make sense just to reposition all the hardware to achieve better geometry and get rid of the dumb-high mounting situation. i'll try to remember to post some pics of my progress here now that i know someone else is working on the same situation.
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