Short-shaft outboard on a Long-shaft transom - conversion

[lantz86]

€£¥

 

[Woodonglass]

Uhmmm, What did you do on the inside to beef up the transom? Extending the motor back as you have done can sometimes changed the dynamics of the boats performance and handling. Did you Core sample the transom to ensure it was sound and had no issues with rot etc..? Have you checked the deck for any soft spots. Boats of this vintage often have issues with soft decks and this could lead to stringers and bulkhead issues. 1/4" Aluminum is not very strong considering the weight of that motor! 3/8 or 1/2" would have been my choice. Did you use Stainless hardware to bolt it to the transom? Did you coat the hardware and the holes with 3M 4200 or 5200 sealant? What did you use on the Treated wood to seal it? Why did you use this approach instead of just cutting the transom down and attaching the motor to the transom? The splashwell could have been cut away and re-fabricated to accomadate the motor. What's your plan for sealing the top of the transom? I give you an A+ for originality and out of the box thinking It's kind of like a non adjustable Jack Plate.

 

[lantz86]

Woodonglass, temporarily i have 2 treated 2x4's on this inside that the bolts go through. I plan to use an aluminum plate in there place. Yes, you are correct about the handling and performance of the boat changing due to this. Most of the time it's actually for the better. Which is why you see some engine brackets extending 24" or sometimes more from the transom on some offshort boats. The deck is perfectly sound and strong. As for the 1/4" aluminum, i consulted a couple experts here in town, and they said it will be find given the weight of the motor. It's actually very strong. Every jack-plate i have seen for 50hp and under is always 1/4 aluminum. And they are 4 pieces, rather than my two solid ones. All stainless or galvenized hardware, Very heavy duty bolts. The holes and hardware, i coated with a 50 years marine caulking. I used a latex, heavy duty paint with a marine additive to coat the wood. Ive used the before on other projects and it works amazingly. I didn't want to refab the splashwell because i suck at fiberglassing. It was just more than i wanted to mess with. And i really wanted to maintain the splashwell because there isn't a scratch on it. Right now i have a thick coat of liquid nails on the cut, but i am going to remove that and fiberglass it (as best as i can). The transom is perfect. No rotting at all. I mean, it had a motor twice and big as this one on it before, and clocked at 53mph with that 115 evinrude... That's just crazy to me. Thanks!

 

[Willyclay]

BTDT but it was long before the internet, iboats.com Forum, etc. and do not have any pictures. I did what Wood suggested and cut-down the existing transom on a 1960 Glasspar Avalon to accommodate a short-shaft Mercury Mark 75. My motor had only one set of mounting holes in the transom clamp, like your motor, so trying to get the "sweet spot" for mounting height was my primary goal. With no set-back on mine, the conventional wisdom then was to get the AVP level or slightly above the bottom of the hull which on my boat was complicated by a fairly prominent keel. Your mounting plate with its set-back will change where the wake contacts your lower unit /AVP. You may suffer from excessive drag or ventilation but mounting height seems to be best determined via trial and error. Be prepared to make some mods. The set-back looks like it simplified the steering connection issues with your push/pull system. FYI, be careful not to bend the steering cable tighter than 12inch radius or it may bind/kink. I think I have a link to a website for plans to build an adjustable jack-plate and post it later if I can find it. Good luck!

EDIT: Here's the link - http://www.dillon-racing.com/jackplate/index.htm

 

[lantz86]

BTDT but it was long before the internet, iboats.com Forum, etc. and do not have any pictures. I did what Wood suggested and cut-down the existing transom on a 1960 Glasspar Avalon to accommodate a short-shaft Mercury Mark 75. My motor had only one set of mounting holes in the transom clamp, like your motor, so trying to get the "sweet spot" for mounting height was my primary goal. With no set-back on mine, the conventional wisdom then was to get the AVP level or slightly above the bottom of the hull which on my boat was complicated by a fairly prominent keel. Your mounting plate with its set-back will change where the wake contacts your lower unit /AVP. You may suffer from excessive drag or ventilation but mounting height seems to be best determined via trial and error. Be prepared to make some mods. The set-back looks like it simplified the steering connection issues with your push/pull system. FYI, be careful not to bend the steering cable tighter than 12inch radius or it may bind/kink. I think I have a link to a website for plans to build an adjustable jack-plate and post it later if I can find it. Good luck!

EDIT: Here's the link - http://www.dillon-racing.com/jackplate/index.htm

Willyclay, all excellent points and concerns. I actually did research the keel and lower unit height topic a lot before deciding the amount of set back. Mine only goes back 6". The bigger motor that was on it before was very close to that amount of set back anyways, due to it's larger motor bracket and tilt/trim motor. I also went to a few other boats with similar set backs, and took measurements. I believe i have it at a perfect height, given the research i have done. But, i am prepared to make mods. That's always a given. The steering works great. It's smooth and responds just fine. Thanks!

 

[lantz86]

Forgot to post this picture earlier.

 

[ondarvr]

Looks too low, could go up an inch or two.

 

[jbcurt00]

Looks too low, could go up an inch or two.

I'd say at least that much. W/ setback, many perform better w/ anti-cavitation/ventilation plate 1"+ above the keel. Looks lower then keel already.

 

[Woodonglass]

I do a ton of research and I'd be grateful if you share yours on the effects of the motor set back and motor height settings. It's all well and good for Ocean going boatsof 20ft or more but it's much different for your average Fresh water Craft and for boats of your Length, Weight and HP. I sure would be interested in reviewing the research articles that led you to this decision, maybe I could add them to my library. I'll be glad to share any of mine with you, if you're interested. Here's one that's on topic...
http://media.channelblade.com/boat_...s/dts/dealerlibraries/8441/copetipstricks.pdf