Need Help - Motor Placement 36" Hydroplane

[AndyMartz]

I have a 36" hydroplane (Ship Ahoy Model) of Atlas Van Lines. I'm plugging along on it but I need some advice on how much forward or back I should place the electric motor. Should I center it over the sponson's or have the front of the motor right at the sponson line (photo with ruler at sponson line).

Any advice from modelers is most appreciated as this is first time I've ever put together one of these.boat.jpg

Thanks much

Andy

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[Dr. Jet]

My approach to such things, and I'm sure many here follow the same process, is to lay out all your components (maybe tape them into place) and move things around until you get the model to balance at the appropriate place. If you plan to add flotation, consider that when balancing things. Remember too that paint adds weight proportional to the area being painted. Don't glue anything until you are 100% satisfied. The LAST thing you want to do is build it, then have to add weight to make it balance.

There is no real fixed rule as to where things go. Use common sense, try to minimize bends in your stuffing tube, think about where the cooling hoses and motor/battery wires will go and strive for a neat and sanitary final configuration. Keep the receiver away from the ESC. Try to keep the weight spread out and as low as possible and keep the motor wire runs as short as possible. Post photos here and I'm certain those more knowledgeable than I will chime in with real-world experience. I mainly build micros, so this is out of my area of expertise, but general rules apply across all sizes.

Is that one of those "Spray 'Glass" hulls being sold on eBay? They have always caught my eye, but I have been hesitant on building one without seeing more. What are your opinions about the hull? How is the fit and finish? What does it weigh? Is it a thick layup or is it thin? Is it square/even? Are the ride surfaces clean and sharp, or do they need work? Is it stiff, or does it need reinforcement? Anything else that jumps out at you?

Inquiring minds want to know.

Please post more photos and please do a build thread.

[Jesse J]

I typically install hardware first, turn fun flotation and all parts that ?have to be there? then go about the above process.
Also consider battery pack size and esc orientation.

It looks like a heavy lay up, but if there is still decent flex, consider stiffening the tunnel with carbon fiber before moving forward with internal components.
Flex causes drag and that is bad for the electronics.

Equally important: Think about what range you want the cog to be.
Not sure on that hull, but if it were me, I?d allow for 1-2? behind sponsons, unless you have recommendations form someone who has built it.

[Dr. Jet]

Jessie nailed it on the flex thing. On the vast majority of my hydro builds I include a bulkhead that goes from sponson to sponson. This really helps lateral stiffness.

DSCN1468.jpg

I understand it may be impossible to get a bulkhead in there after the hull pieces have been joined. In my MHZ miss Madison (pre-joined hull), I ran a carbon fiber rod from sponson to sponson at the very leading edge of the tunnel. This HAS to be firmly anchored at each sponson.

The formers that run lengthwise help reduce twisting when attached to the upper and lower hull halves. They act as shear webs.

A lot of people reinforce the floor with carbon fiber. This, along with the other structural members, really helps to stiffen things up.

DSCN1632.jpg


EDIT: I looked at that hull on eBay and it appears the necessary structural members are included with the hull. Properly anchored, they will make your hull as stiff as a 2x12 floor joist. In the eBay photos of the un-joined hull, the spray 'glass looks fairly light. I'm sure many people here would want to know the empty weight.

[jkflow]

Just went through this on a similar setup. Like everybody else stated, do a mock-up and place the CG right.
See where that places your battery as the heaviest item. I was able to place mine in front of the motor but they could have also been in the sponsors with the motor further forward.
Next thing, you have to worry about adjustability of the strut, leave the stuffing tube long enough to allow for adjustments when glued in place. That will also determine motor placement.

[AndyMartz]

Thank you for your reply.

Yes, its one of those “Spray Glass” hulls sold on eBay made by Ships Ahoy Models out of Michigan. If I had to do it over again, I’d say no…I wouldn’t buy one. The hull is made with matting, probably 3 layers, no weave or roving with grey gel coat. It’s pretty “wobbly”. Fit and finish – mediocre, hardly clean or sharp. Since I purchased this hull (not knowing what I was doing) I’ve seen other boats for quite a bit more money but they are superior to what I have. Square and even – no. Had a very hard time putting the two halves together and get everything to match. I put a 20 pound hand dumbbell on the bottom but the top and bottom molded pieces did not exactly match together precise as they could have been. Weight, is 5.5 lbs currently, no running gear, no rudder, no motor. I thought the bottom was so thin (where I am placing the strut) I decided to reinforce the rear of the air trap with a thin piece of aluminum on the bottom and matching piece of 1/8 inch plywood on the top. The hull does come with 2 rough cut ? plywood longitudinal for frames fore and aft, a rough cutout transom and arching sponson frame. I say “rough cut” because they are hardly precise. Overall it’s not for a beginner nor would it satisfy a season rc racers (IMO).
Thank you very much for your balancing method. That is exactly what I needed. I’ve never built an rc boat before and this is taking me longer than I thought, no instructions, no one around this area that is into rc hydroplanes which is odd because we have the big piston powered vintage unlimited come to town every year for a festival called Mahogany and Merlot plus the limited hydro’s race.
Andy
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[Alfa Spirit]

Where must be the COG on an hydroplane hull ?

[Dr. Jet]

On something that size, it should be 1"~2" behind the sponson.

I'm sorry Andy isn't real happy with that hull, but his response is somehow exactly what I expected. It's a tough call, but I might suggest abandoning that hull and building something a bit more appropriate for someone who has only a few builds under his belt. If you could find a used Pro Boat UL-19 you would be far better off. Here's a new one on eBay that can be at your door for under $500. All you will need is a radio and batteries.

https://www.ebay.com/itm/174909122581?hash=item28b9656815:g:~FcAAOSwsR9hJrkQ

EDIT: I would have suggested you get one from Steve at OSE, but they are currently on backorder.

[AndyMartz]

So….still working on this fiberglass 36” hydroplane. My current dilemma is mounting the rudder. Since I put the skid fin on the right side, I decided to mount the rudder on the left….since that is what I’ve gathered from posts I’ve read. So question: how far should the rudder be placed from the centerline (strut)?
Haven’t seen a real good photo of the rudder placement in reference to the strut. Is there some advantage to having in close to the strut or somewhere in the middle - from the centerline strut to outside chine? Any thoughts would be appreciated.P3190018.jpg
Thx

Andy
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