absolute beginner Motor and prop fitting

[Brian MuirParticipant @brianmuir84327]

Hi everyone, I am building the (MAR2556) Gwen M from the very basic plans with no instructions. The plans give an alternative section of the keel if a motor is to be fitted. I want to do this and I assume I'll drill a hole to take the shaft. The motor appears to fit below the water line so water tightness is needed. Where can I find a step by step process for selecting the right parts and fitting them. I assume the prop shaft fits into a tube the fits into the hole in the keel. How do seal them all off without preventing the shaft from turning freely. I have looked at other posts but they seem to assume a certain amount of knowledge which I don't have yet.

Cheers, Brian

Edited By Brian Muir on 29/07/2013 18:05:55

[Brian MuirParticipant @brianmuir84327]

Scratch built boat – what to look out for

[LARRY WHETTONParticipant @larrywhetton68737]

HI ,Brian

can not find plan for refrence Larry..

[Ian GardnerParticipant @iangardner62867]

Brian,

It seems a shame that your post should go unanswered so I'll kick things off. I'm not sure how far you have got with Gwen M, but I find it easier to make provision for the shaft and tube whilst building the keel -ie cut a slot for the tube to fit in rather than trying to drill a hole after the boat is built. This may entail leaving a small tab at each end of the slot to keep the keel together whilst building is in progress- these are removed when everything is supported. Hope this is clear!

With regard to fixing the tube in the slot, people generally use car filler paste (Isopon) or thickened epoxy- I prefer the latter for some reason. I use Zpoxy with some colloidal silica or low density filler. The Isopon is probably easier to source.

There are a plethora of tubes and shafts on the market. For something like this you may just need a stern tube of the right length with bronze bushes and a steel shaft- probably threaded M4 each end and a suitable coupling for your motor and shaft.

With regard to the inboard end of the shaft being below the water line, it might help to fill the tube with grease – vaseline or grease sold for the purpose. I generally find it doesn't, and some ingress may be inevitable. It is possible to make a gland which will keep water out, but it may be more complicated than you want at this stage. Our own Mr Abell had a drawing for such a thing, if you look in his album.

Suppliers you might find useful are Model Boat Bits and Cornwall Model Boats. These are two I have found very helpful but, of course, there are many others.

As with all of these things, there is more than one way to skin a cat, but I hope this points you in the right direction. I'm sure others will chip in (shoot me down in flames) in the fullness of time.

Good luck….Ian

[ashley needhamParticipant @ashleyneedham69188]

Ian, I think you have it there. To add to this good advice, and pardon if you know this already:-

Position your motor, make allowance for the length of coupling (ask supplier, or see specs) and then measure from end of coupling to prop position. Note lengths of prop tube are what you order, size wise, the actual shaft inside will be one inch longer to allow prop and coupling to be fitted, but this is how they come. A 4mm shaft should be ok unless the length exceeds (I would say) 10 inches, then use a 5mm shaft,

Not sure what size motor you need, but a mount to take a 500 can size engine will allow fitment of a whole range of engines, from low drain 540`s to 555 or 600 types and be a good starting point.

Dont forget there is no such thing as overpowering a model, thats what the throttle is for!

More is less (power and worry respectivly, when the bank is looming fast.

Ashley

Ashley

[Brian MuirParticipant @brianmuir84327]

Hi there, and thanks for your prompt and helpful responses.

I did see a post that took the pragmatic approach and suggested enabling access for a syringe to be used to dry it out. That might be best?

Ian's description was very clear, so I think I'll go with that, I am just copying the plans at the moment before marking out and cutting the parts, so I will drill the hole in advance.

What is the coupling you have mentioned – the connection between the motor and the inboard end of the propshaft?

I take it the prop is threaded to fit on the outer end of the shaft.

Ashley, with regard to motor size… not sure what " a 500 can size engine will allow fitment of a whole range of engines, from low drain 540`s to 555 or 600 types and be a good starting point." means.

Do I need to know the weight of the model or something like that to choose the motor? What does "low drain" mean?

What about battery packs – do I have to worry about the size of them for different motors?

Sorry if these questions seem a bit obvious, I am going to join a local club so I can pick their brains, but there's a lot of knowledge on this forum to tap into too.

Cheers, Brian

[Colin BishopModerator @colinbishop34627]

Just anothr quick point – when you screw the prop onto the shaft, be sure to use a locknut and also a washer between the locknut and th tube to help seal against water ingress.

Colin

[Malcolm FraryParticipant @malcolmfrary95515]

If its this one – **LINK**

then its about 30" long. To provide realistic auxiliary power for this type of boat a 385 motor (15 volt type run on much less, say a 7.2 volt pack) would be appropriate. It isn't a racer. A low drain motor generally means that it doesnt pull much current. Usually it also means that it doesn't rev at a high rate, but does produce good torque, ideal for a prop that is going to move a hefty boat (like this) at a modest speed, rather that a light boat at great speed. A 540/545/550/600 type would probably be overkill, and in a displacement hull, probably not do that much extra good. The 35 and 54 refer to the motor can length. See the thread about motor sizes for deeper confusion.

A larger battery pack will generally heavier. Archimedes principle applies – you can't put a big battery in a small boat. For any given voltage of battery, a wide range of capacity is available. More capacity, longer run time, Sadly, also heavier, so there are limits. Not all batteries can deliver the current required by any motor, so this is another area where size does matter.

Coupling. There are some rather nice ones on SHG's site, about page 21 on their catalogue.

The prop is threaded, but is is sensible to use a locknut. You want to keep the prop after reversing. One of Newtons laws applies here – the prop will try to unscrew itself when you chuck it in reverse.

Another approach to the inevitable water weep via the shaft is to arrange a sump to catch the drips. It does make it easier to catch the water for syringing out once it has found a way in. Probably why so many stands have a selection of syringes at shows.

In old time converted sailing boats, it was not uncommon to use a very hard wood such as lignum vitae as the bearings and to stuff the space between (hence the term "stuffing box" with a lead based grease to lubricate and help keep the water out. Fortunately, we can get prop shafts and tubes ready fitted with bearings from a shop.

[Brian MuirParticipant @brianmuir84327]

Thanks Malcolm for the link, I should have included that…. and for the info, it is starting to make sense now.

The modle is shown below, i wonder if anyone else has built it (should I start a new thread?)

Cheers, Brian

Gwen M

Model Boat Plan: MAR2556

A 1:12th scale schooner rigged sharpie sailing yacht, based on the American skipjack style boats. The plan shows all frames and internal structure which follows fullsize practice as far as possible, and also dimensions of the sails and rig. Model hull is 720mm loa by 210mm beam. Designed by Ian Sharpe.

Featured in: MM July 1996

[Author]

Posts