inboard to outboard conversion

Discussion in 'Fiberglass and Composite Boat Building' started by billy67, Oct 19, 2012.

  1. billy67
    Joined: Oct 2012
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    Location: malta

    billy67 Junior Member

    Hi all, this is my first post here, i own a 22ft grp boat which has a planing hull and for these last twelve years has been fitted with twin diesels, guess time took it's toll on the poor things and i decided to opt for an engine change an do a conversion to outboard i was told that the keel aft has to be taken off to about a metre and a half from length as it will effect the outboards' performance anyone can give some ideas how i'm going to fill the gap between the keel and what is the best material i can use, is it possible to work just from the outside as the keel is filled right up from the inside with resin mixed sawdust and glassed from the inside too, i have posted a pic of the aft section.
     

    Attached Files:

  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Can you post another photo showing this area from astern? I suspect this is just a molded in skeg, for directional stability and can be removed. Ideally, you'll want to make it flush with the sections just forward of the skeg, making the "keel" dead straight, instead of canted down as it currently is.

    To remove it, just cut it off with what ever you might have, though a reciprocating or a jig saw will work best. It can be repaired from the outside only, but it's better to work from both sides, for best strength.

    The best materiel to use is epoxy resin and biax fabrics, though for this sort of thing you certainly could use polyester, mat and roving as well.
     
  3. tunnels

    tunnels Previous Member

    ok the keel needs to come off so the hull is flush and a little longer than just 1500 on a boat that size More like 1.8 to 2 .0 mtrs is better !!
    You have a glass hull so all your glassing can be done outside . from the place where you cut the keel grind back at least 200 mm all round all edges no sharp corners and you will need Chopped strand matt, resin and woven roving , and gelcoat !! the back end of the keel is best to be cut about a 30 degree angle facing forward the grind up is very important to make sure there is no paint , no gel coat and that its all a minimum of 200 mm all round not just parts of it !!!!. Resin !! the boat will be made of polyester resin and glass . a good quality polyester is ok or look and see if you can get Vinylester resin . its simular to polyester but stronger and stricks better . If you get talked into using epoxy Think really hard do you really need it ?? its exspensive and you cant use polyester or Vinylester over the top it dosent stick . You start with epoxy you have to stay with epoxy all the way through the job !! . Post some picture of the boat outside and inside because you will have prop shaft holes to fill over inside and out Plus rudder shaft holes and the water pic ups for the old motors . with the repairs some is bestdone inside and out side for the sake of safety !! Pictures lots pictures and lots details in them and we can help you much much better . :p


    you fitting twin out boards ??
    what kind of horse power ??
    with out seeing what the boat looks like i would say a pair of 150 hp would be the smallest you want to go .
    200 or 225 even 250hp each would be better . not much differance in the overall weight but the 250 extra long shaft would be my choice .
    Its good to have extra power that you dont need to use all the time than to have smaller motors that are working hard all the time !! the exrta long shaft is a better move to get the motor away fron the sea behind the boat !!.
    so throwing inboards out this is a chance to get rid of everything else thats not needed as well .
    your transom will need some serious rebuilding thats for sure , you can fit a bracket complete with a boarding platform !!
    The complete unit can hang the two out boards and gain all that extra space inside the boat !! a few things to think about and look seriously at !!!! :D
     
  4. billy67
    Joined: Oct 2012
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    Location: malta

    billy67 Junior Member

    hi all thanks for all the info i appreciate a lot i am going to open from the inside too for safety's sake that way i can have better access, here are a couple of photos from aft view, sorry i forgot to mention that the outboards are going to be fitted on a platform that is going to be fixed to the transom so i will keep the space i have at the back as you can see the boat is full of openings which i have to fill,can you specify what is the best way to fill machined holes that are below the waterline please.
     

    Attached Files:

  5. tunnels

    tunnels Previous Member

    Let the work begin !!

    Ok
    Looks like a slow boat for where ever!!
    Yip lots holes to fill !!
    Best to filled from both sides ,inside and out . when grindng dont be afraid to make the patchs big , gets better bonding and glass to glass is the way to go rather than filling holes with filler and a scim of glass over it One good bang and it all falls out !! You are going to have fun with cutting the keel a grinder with a cutting disc will do wonders and makes clouds of dust i warn you !! dust for miles !!.
    The cockpit drains are at or close to water level ?? Show us whats inside the cocpit at the back !! it could be better to glass completely over those drains and cover and not use at all . remember the motors are outside so the drains could be awash even with less weight in the back !!.

    So what size motors you looking at ?? like i write before dont underpower the boat !! you will live to regreat it for sure !! and look seriously at 25 inch shafts and get the motors high up out of the water . They like clean air to breath , cuts down on corrosion and a lot less looking after !!.
    Lots more pictures inside and out . A picture of the whole boat side on to see what you got !!. :p:D:p
     
  6. tunnels

    tunnels Previous Member

    repairs

    The holes in the transom ! have to grind the glass back and clean out the hols so can see what there . Could be glass wood glass . is best to add wood where there is wood already and epoxy it in place hen 200 mm around the hols grind the glass on a tapper into where the wood is

    Holes in the hull will be a little differant because its all glass need to grond 200 mm all round insdie and out and they will be glassed totally . I will find some drawings for you to get the idea what to do !!!!
    Back soon !!:p
     
  7. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    What is the structure of the transom and stringers like? Most inboard boats have a transom that is not designed to withstand the thrust of the engines. Usually they are maybe 6mm of fiberglass or so in a boat that size.
     
  8. billy67
    Joined: Oct 2012
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    Location: malta

    billy67 Junior Member

    The transom is about 10mm thick and it's only fibreglass i intend to reinforce it with marine plywood and glass them as the outboards and boarding platform will be bolted to it, what thickness should i get the plywood to? and the stringers are timber two inch by eight inces high reinforced with fibreglass i will post some pics of the cut keel and the shaft log holes tomorrow.
     
  9. whitepointer23

    whitepointer23 Previous Member

    if you are building a glass outboard pod you can extend the stringers through the hull so they become the back bone of the assembly. the other advantage is you can mould the pod to match the hull giving extra bouyancy to carry the engines, you just taper it so it clears the water on the plane. you want 1 1/2 to 2 inchs of transom thickness i would think, but the experts will advise on that one. also glue in large triangle ply gussets from the stringers to the inside of the transom.
     
  10. tunnels

    tunnels Previous Member

    I completely understand what you saying but depending on motor size its better not to make the pod as a hull exstention as what you gain in reality could have a slowing down effect on the hull plus will change its water line length and change the way the boat behaves in the waves and out on the ocean !!. Not all coast lines have wave that are the same in every place on earth . :(
     
  11. whitepointer23

    whitepointer23 Previous Member

    most pods i have seen the last few years have been hull extensions for bouyancy but they taper up to the rear so they clear the water on the plane. some have only been about 2 ft wide but still follow the hull shape. those ones had swim platforms each side which looked pretty good. a well designed pod conversion makes a good boat better, a bad pod conversion can turn a good boat into a bad one.
     
  12. tunnels

    tunnels Previous Member

    Floatation is ok and a good option but not adding onto the length of the hull as such !! you have to make sure the pod is well away from the water surface exiting the end of the hull or it will cause sucktion and untill the air gets under it your in trouble it will have a tendancy to suck the back of the hull down down wards and you need lots horsepower to get out of that situation and get going . Its happened many times before and will keep happening to people that dont understand what they are doing . !! Look at exisiting ones that are out there in the market place and learn from them !! :(
     
  13. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Adding to the hull will not likely cause much difficulty on the usual shapes employed in powerboats. As far as the pod being well away from the hull and this suction stuff, well maybe you should look into hydrodynamics a bit, before making these kinds of statements. This is an old wives tail that's been long disproven, though it does require you keep up.

    It's important to compare apples to apples. Pods are different and should be designed differently then hull extensions. Pods generally have a step, after the hull, with sufficient buoyancy to over come excess squat, (for whatever reason). The step acts to "shear" the flow clean, so the props have something to chew on and the buoyancy to help counteract the added leverage gained when the outboard(s) got move a few feet aft. There's no suction as you might think. If you drilled holes in the bottom of the pod or aft sections of the boat (it's been done), water would shoot up through the holes like mini fire hoses on full.

    As to hull extensions, these tend to be more difficult to design, because the balance of the boat can be altered, but in most cases, again with the typical shapes employed and assuming reasonable power application, few difficulties if the after sections of the hull are extended straight back.

    Transom thickness is dependent on bottom loading, which is partly dependent on how big the engine(s) will be. How much total horse power are you tossing at this transom? What is the target speed and weight of the boat?
     
  14. billy67
    Joined: Oct 2012
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    Location: malta

    billy67 Junior Member

    250 hp is the aim and i calculate 30 to 35 knots w.o.t will be the target speed and as for weight i'd say about 1700 kg, i'm guys plenty of work to do this morning grind grind grind grind soooo much to grind:eek:
     

  15. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your transom should be 2.75" (70 mm), including the inner and outer 'glass skins, plus for this HP and weight, should be well supported and braced, particularly if bracketed.
     
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