Custom outboard: 15-40hp w/25"+ Shaft?

So to follow up on an earlier post- I was formerly occupied with finding the highest power to weight ratio. This is still an important specification, but I have refocused my search on finding an outboard that has an uncommonly long shaft- 25" or higher. A search term I have been using with this is XL which I have come to understand is any shaft longer than 20". I have found one matching motor in the USA that fits this spec (measured from the transom mount to the bottom of the cavitation plate, which is rated at 35hp and around 118 lbs.
I will probably buy this motor as a good starting point. I have found that 25" shaft outboards are available either in lower than 15hp motors, or higher than 100hp. Nothing really in between.
Considering that situation, it occurred to me that perhaps I could get one of the smaller motors at 25" (or 30" if it were available) and swap the powerhead to a higher HP.

Of course I have some structural concerns, but I also wonder how difficult that would be. It could be that some manufacturers have a similar mounting setup so it might be easier. eg. It would probably be easier to swap a 289 ford automotive engine with another ford 351 Cleavland engine than it would a Honda engine.
Structurally it might be better to get a 100hp engine and reduce the powerhead to save some weight. Anyway, I just wanted to post some of these ideas and see if anyone has any feedback on that before I went ahead with the best factory XL I could find.

Thanks for any help!

 

Hi Salukikiev,

The lower unit of a 15 hp outboard won't survive if asked to handle the torque of a 40+ hp powerhead. It would literally tear the gears to shreds. And, in any case, it wouldn't be able to fit a large enough propeller.

I would suggest that your best bet is to design your transom and motor mount according to standard dimensions for the power class you're considering.

Your second best bet would be to mount the motor on an adjustable-height jack plate. This is common practice among the speedboat crowd.

A desperate third choice would be to get a 20" shaft motor, separate the lower unit, and fabricate custom extensions for the drive shaft, water pipe, shift linkage and shaft leg.

 

Not sure what youre looking for, but both the Honda 50 and 60 hp motors come in 25 in shaft. These motors are popular with comercial fishing boats and pontoon boats. For additional choices you might check out what the pontoon guys are using.

http://marine.honda.com/Products/modeldetail/BF50

 

hey thanks for the replies!
40HP is the upper limit of my target. 25hp is more practical and would represent just 10hp more than the 15's frame and mechanics- probably within the safety margin for a 15hp motor. I'm not opposed to re-engineering the thing and I have the equipment to do so, but it subtracts available time from primary goal, so if I can find an off the shelf or quick solution to this, then I'll save myself a lot of time down the road, and probably improve reliability.

Anyway, the motor will be fully integrated to a custom frame, so an adjustable jack plate isn't going to work. I'll likely wind up with a custom made jack plate- this motor will be fixed in all directions- it won't need to pivot, so I may try to ditch associated pivot/hardware to save some weight.
Your choice 3 is closer to what I had in mind. I can fabricate a custom extension if I need to. It sounds like my best bet is to start with that 35hp 22.5" XL engine unmodified and see if I can make it work. If not, I'll probably look into fabricating an extension. If I was going to go to that much trouble, I'd probably take it higher than 25" though. I guess the 1st step is to acquire that 35XL, so I'll take a look at that and see how it goes! thanks for the help!
-k

 

that IS a pretty near miss! thanks for the link! Unfortunately it's about 100lbs, and 10hp over my target(s). On this older link- the author mentions relatively easy conversions of Johnson/OMB motors. I wouldnt' mind finding a nice kit solution, but so far I haven't come across it.
Also, I'm financing this experiment myself, so splurging on a brand new motor isn't necessarily my 1st priority. The XL shaft 35hp i mentioned is only going to cost me hundreds instead of thousands, and I'm ok with that!

 

I believe that the old two stroke 25hp Johnson came in 25in shaft. Perhaps even the 25hp yamaha. Best to go to an outboard shop , run by an old timer, and ask about older model choices.

 

Thanks again!
It would be great if you had a model or search term I might use to hunt one of those down!
I did try a query at http://www.outboardmotor.net/ and everything 25" was listed as either 9.9hp or 100+ hp. the 30" search only yielded results of 250+hp.

Part of the trouble is entering the quotation mark to denote inches- most search engines ignore it and return results with 25 horsepower or some other mention. I'm stuck using less common (25in) terms and returning less results- so any specific terms would be very helpful. I've recently had some luck with XL (and rarely- XXL) for example.

 

Check with these guys. And why do you need such a long shaft ?

http://www.boats.net/outboard_motor/Yamaha/25HP/Y-25MSH.html

 

Yamaha has a 25hp 4-stroke model FT25FETX (25.6"), but I think it is electric start/remote control only, unless I am reading the specs wrong.

 

hey- thanks! That's a great suggestion, and the specs I found agree with you! It's literally the 1st (only?) factory example of a 25hp engine with 25" shaft that I've discovered! (well.. you've discovered!)

The only drawbacks to this particular one are of lesser, but still significant importance. Those are:
1. It's a 4 stroke- which would be fine if not for the power/weight ratio. This motor is 170+lbs. The other 2 stroke's I was referencing generally come in at >120. 50lbs isn't generally a big deal for most boat applications, but this one is particular.

2. It's pricey! I don't mind investing for the perfect motor, but I'm not sure it would be money well spent in this case, and I'm financing this whole build independently. Further, I will very likely permanently modify this engine (mounts)for my purpose and thus the resale will probably be effected. It's easier to risk an engine that I only spent $1k on than the $6-11k I was seeing that one listed for. I expect I'd also be paying for a warranty that would be quickly voided.

Anyway, so I really appreciate the suggestion and welcome any others or ideas, but I will probably hold out a little longer as I have the opportunity while designing other aspects for the moment. I just thought I'd take the opportunity to clarify some of the priorities I have.

Thanks again!
-k

 

It would be within the ability of a lot of machine shops to custom extend legs of old 2-bangers in your hp range, but the bill at the end of it would be pretty hefty I'd think, driveshaft, shift linkage, water tube and the alloy leg insert, plus new stainless screws to hold it together. Don't know you'd save a lot and be left with an orphan no-one would want if you sell.

 

It's true- I have been thinking that the only way I'd ever find this oddball combination is to fabricate my own extension/components. I'm pretty sure I could pull this off as I have a well equipped machine shop and CNC available in house. However, if I could find that combination ready-made I could save all that trouble and hassle and stay focused on the rest of the design- which is entirely custom. Not trying to re-invent the wheel if I can find just the right wheel with some extra research.

Anyway, for now, its not urgent, so I can just keep my eyes peeled until I absolutely have to nail down the motor. In actuality, I could probably get away with a typical lightweight 20" long shaft, but it just wouldn't work as well.

Thanks again for the help!
-k

 

The main issue would be the integrity of the extended shaft, I'm not sure how much of the original strength might be lost by welding in an insert. In some instances, it has been done with a short splined extension, so two pieces instead of one. Would probably be a good idea to torque test somehow before hitting the water, they are dead in the water with a broken driveshaft.

 

Incidentally- I did find a pair of guys near Dayton, OH who I had originally contacted about their XL 35hp 2 cycle- they had advertised it as checked over and running, but in actuality, it evidently is not. Now they have come up with an XL 25hp, but they're terrible about communication, and the simple fact that they would have not one but TWO motors that happen to be mid-power XL shaft engines is pretty hard for me to believe. Especially after combing through every classified I could find nationwide.

If anyone in the Dayton/Vandalia area of Ohio wouldn't mind checking these out to validate that either of these are legit, I'd be glad to pay a finders fee.

 

Would you necessarily weld in an insert? I had read that OMB and Johnson have kits available. I didn't think there was welding required- and I was thinking they would probably be hesitant to offer a kit that unreasonably compromised their integrity.

That said- there is a lot of welding going on with this project. I've gotten pretty good with a TIG, but I'm more concerned with metallurgy issues in regards to the outboard. I think I can trust my frame, but not as confident in a welded outboard feature.