I/O to outboard

Discussion in 'Boat Design' started by Rranger, Nov 26, 2020.

  1. Rranger
    Joined: May 2013
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    Rranger Junior Member

    Sorry for the long winded post. I have a older sangster 19 ft. I took a Chrysler slant six and Volvo drive off and plan on adding a pod and a 125hp 2 stroke Merc. I removed the small amount of rot and am ready to start glassing. Right now there are a 1/2” sheet and a 3/8” sheet on the transom less the enlarged old penta drive hole. Those two sheets and stringers and old engine mounts are solid as a rock. Plan is to build the transom up to 2.75 inches thick including a 1/4 inch plate on the inside and outside of the transom, with aluminum bracing coming from the old engine mounts and attaching to the inside plate and bolted through the transom connecting the outer plate and the outboard engine all together. I have a couple questions. The remaining plywood on the transom does not want to come apart if I try to separate only the 3/8” from the 1/2” to create a lip to offset the joints to patch the hole. Would it be safe to laminate same sized pieces of the 1/2” and 3/8” together and then glass them to the outer fibreglass skin to cover the hole and then add at least three pieces of 1/2” plywood to the inside of the transom and the two outside metal pieces.? I would also have to cover the old hole from the outside after glassing the inside.
    And the motor I have is a long long leg which I had never heard of before and has a 22.5” shaft length, I would rather have a 25” all pod users seem to have, but it’s what I have. Will that 2.5” matter setting up the pod height on the transom or is it advisable to get a 25” shaft engine. I also question the 125hp’s ability to provide decent speed and power? Anyone up on hp required for that 19’ Sangstercraft? The inboard in it previously the slant s8x put out 145 hp from what I can find. Attached a couple pics. Thanks
     

    Attached Files:

  2. gonzo
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    gonzo Senior Member

    There are several issues you need to address. The first, which is usually very difficult to solve, it the shift of the center of gravity aft. That will make the boat squat. Trim tabs may alleviate some of the problem. As far as decent speed, that is something you need to quantify by specifying a minimum. The length of the leg is not critical, however, the height of the antiventilation plate is.
     
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  3. Rranger
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    Rranger Junior Member

    Thanks for the reply. I should have mentioned the trim tabs are sitting on a shelf ready to install. I am just wanting reasonable speed and power for a max trip of 5 miles kind of one way fishing trip. All inland lakes not far from home. I have no idea of speed needed just get up and plane. Just wondered if anyone out there podded a similar sized boat and what engine they put on.
     
  4. fallguy
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    fallguy Senior Member

    Laminating plywood to the exterior skin.

    no

    Take some pictures. Glass is always the final layer, then fairing, then coatings.

    For the pod and your odd leg length. I'd get a manual jackplate and push the engine wherever you want.

    Make sure the pod manufacturer can help you with the pod locating. Do it wrong and you'll have trouble with drag or interference, etc. outside my knowledge area...just know enough to caution
     
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  5. JSL
    Joined: Nov 2012
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    JSL Senior Member

    Also make sure the prop blade area of the new O/B is similar to the old I/O. Example: If you had 16" diameter 3 blade on the Volvo then a 14" 3 blade on the Merc might not be sufficient.
     
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  6. Rranger
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    Rranger Junior Member

    I don’t follow you about the lmonating my plan was to laminate a 1/2” piece to a same sized 3/8” piece and then laminate that piece to the cleaned exterior glass in pic two blocking the hole. Then on the outside, glass in the missing exterior piece covering the wood to the thickness of the rest of the fibreglass.
     
  7. Rranger
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    Rranger Junior Member


    Had never heard that thank you JSL. I’ll have to ask the guy I sold the I/O to.. it was a a Volvo 270 drive.
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    That looks like a 20" motor to me, where are you getting the 22.5" dimension from ? Those old straight four Mercuries are a strong motor, I would say it will do the job, what prop pitch do you have ? 17" would probably be the right size for that. Obviously 25" shaft length gives a bit more security, but this is not a real deep vee by the look of it, so you should be able to get away with it. Depending on your pod, perhaps.
     
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  9. Rranger
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    Rranger Junior Member

    I measured it from the cavitation plate to the transom bracket. I’ve measured a few. Phoned Seaway Marine in Seattle asked the parts guy why I was getting a 22.5” measurement and he’s the one advised me Mercury did make what they called a long long shaft back then and that’s what it is. And the motor did not come with a prop, will have to find one.
     
  10. ondarvr
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    ondarvr Senior Member

    You don't have much of a choice on prop diameter. You go with what's available for the motor. The current prop probably won't be what you need.

    These old boats used the same hulls for I/O and O/B, balance shouldn't be hard to achieve.

    Transom height is 20" and 25", but outboard manufacturers don't always go by that exact Length. If it's 22.5" you have a 20" shaft.

    Don't worry too much about how strong the laminate is where it's covering the outdrive hole. The pod will typically spread the load wider than the hole, so it may see little to no stress.

    Are you using an actual flotation pod, or a bracket?

    Adding braces from the transom to the stringers isn't a bad idea, but don't overdo it.
     
    Last edited: Nov 27, 2020
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  11. Barry
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    Barry Senior Member

    would not the prop operating rpm and respective horsepower have to factor into this?
     
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  12. fallguy
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    fallguy Senior Member

    Sorry. I didn't realize you were going to glass over the core plugs.

    Personally, I would rout out some of the old ply around the keyhole.

    Take better pictures; get better help.
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I know there are engines with slightly longer than the 20" distance as you measured, they still call them 20", in any case you are going to use the engine, so doesn't matter, you just adjust it to the most height that allows full use of it. I can recall that a lot of the engines that were designated short shaft, for 15" transoms, would measure at around 16.5 or 17 inches from the bracket to the cav plate. That engine I think had a 2.08 to 1 reduction, I would say 17" pitch stainless would be the go. Unless your engine has the V6 box with a bigger reduction, I don't think they did that though.
     
    Last edited: Nov 26, 2020
  14. brendan gardam
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    brendan gardam Senior Member

    The prop that achieves full rpm or close to it is what the outboard needs. Also you may pick up quite a few knots in top speed. Even a 115 would blow the old volvo away.
     
    Last edited: Nov 27, 2020
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  15. brendan gardam
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    brendan gardam Senior Member

    This boat is 17 ft. It did 36 knts with a 350 chev and bravo leg. The prop was huge. I replaced the i/o with a 135 merc and now it does 40 knts. But I had porpoising problems, I think it was the crappy pod i used, should have gone with hull extension.

    20190413_185041.jpg
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