what would happen if a slap a 10hp, 100lbs outboard on Hobie 18, on the rear cross member?

[Squidly-Diddly]

they say 3hp max and that is good for 5mph, but its some bracket that extends way back, maybe because that is better handling?
they also say you can't put a heavy motor due to lack of buoyancy toward rear, but seems like 100lbs could be counter balanced.
How about adding a 3rd spar at the very stern, if that is needed for good handling. How would it handle if the motor is fixed and only rudders are used.
they say Hobie 18s will do 15mph easy and often 20mph or more, so wouldn't that work with a more powerful outboard?

part of the reason I'm asking is its likely I'd be able to get a cheap/free 10hp, that could be useful on a aluminum runabout, but not a cheap/free smaller motor.

 

[montero]

100lbs it's heavy weight for Hobie 18 . On the rear cross , its ok but you need to compensate twisting momentum .

they say 3hp max and that is good for 5mph, but its some bracket that extends way back, maybe because that is better handling?
they also say you can't put a heavy motor due to lack of buoyancy toward rear, but seems like 100lbs could be counter balanced.
How about adding a 3rd spar at the very stern, if that is needed for good handling. How would it handle if the motor is fixed and only rudders are used.
they say Hobie 18s will do 15mph easy and often 20mph or more, so wouldn't that work with a more powerful outboard?

part of the reason I'm asking is its likely I'd be able to get a cheap/free 10hp, that could be useful on a aluminum runabout, but not a cheap/free smaller motor.

Try to transfer bending thrust outb force into front cross.
Serious weight and power for hobie 18.

 

[SolGato]

Who is they?

Sounds like you are talking about the rating for an outboard accessory bracket.

The reason they are usually set back is to allow for clearance for the tiller so you can still sail the boat easily, and with the brackets being light duty and acting as a lever on the beam, a smaller outboard is recommended due to low weight and thrust.

If you are converting a Hobie to a motorboat, mount what you like within reason.

Keep in mind you might need a long shaft motor, and will have to fabricate your own mount and may want to add a stringer to tie the front and rear beams together to transfer the twisting load on the rear beam.

The rudders would work fine with the motor fixed but clearance with the crossbar as described above might be an issue.

I’ve mounted and seen others mount motors that large.

You might want to keep your eye out for 2-stroke which will save you a bunch of weight.

 

[philSweet]

It works fine. If it's an old boat, and it always is, remove the rear crossmember castings and clean them up and bolt them back together. Build a motor bracket mount that connects both crossmembers and positions the motor well aft of the tiller tie rod. The 21 cruiser is a better starting point, but you work with what you got.

 

[SolGato]

Some inspiration for you…

This is a Honda 8/9.9/10 mounted on a Hobie Wave.

I designed the conversion so the boat can easy be returned back to sailing use with the main clamp brackets remaining in place on the cross beams, and so that the stringers when installed double to act as hand and foot grab bars, and hold storage bins and the fuel tank.

 

[Squidly-Diddly]

Some inspiration for you…

This is a Honda 8/9.9/10 mounted on a Hobie Wave.

I designed the conversion so the boat can easy be returned back to sailing use with the main clamp brackets remaining in place on the cross beams, and so that the stringers when installed double to act as hand and foot grab bars, and hold storage bins and the fuel tank.

thx, thats about what I was thinking of doing. Probably make removable additional cross beams at extreme bow and stern, and bolt them to the hulls with permanently installed threaded inserts. Then optional plywood, with additional wood framing attached underneath the plywood to make a flat platform over the cross members.

Don't have the boat yet, just thinking the possibility of having a well powered big flat platform boat for all-around utility would justify getting one. Really should've been a Factory Kit, with easy DIY wood plans, allowing your choice of plywood grade/thickness, etc, with a few items like the threaded inserts baked-in at the factory. I don't think that would hurt the rest of the boat as a competitive racer. They are a dime a dozen on SF Craigslist, trailers included even.

Here is a thread of a guy that did some mods on a Hobie 18.

https://www.boatdesign.net/threads/hobie-18-tri.63632/page-3​

 

[SolGato]

thx, thats about what I was thinking of doing. Probably make removable additional cross beams at extreme bow and stern, and bolt them to the hulls with permanently installed threaded inserts. Then optional plywood, with additional wood framing attached underneath the plywood to make a flat platform over the cross members.

Don't have the boat yet, just thinking the possibility of having a well powered big flat platform boat for all-around utility would justify getting one. They are a dime a dozen on SF Craigslist, trailers included even.

Here is a thread of a guy that did some mods on a Hobie 18.

https://www.boatdesign.net/threads/hobie-18-tri.63632/page-3​

You’ll need to watch the overall weight, not just the placement of it, if you’re going to use any of the Classic Hobies, especially when you start adding plywood and additional supports.

The newer rotomolded plastic boats like my Wave and the Getaway can float more weight than any of the classic models and are a much better foundation to build on.

Another big benefit is the beams are straight and not curved like older designs, so easy to add a flat deck.

The forward trampoline is also a nice addition on the Getaway as it’s light and doesn’t come with a big weight penalty and is plenty taught to stand on.

Also look out for a Prindle 19, not the older asymmetric ones but newer. They also have decent volume and straight beams.

There has been a set of Hobie Getaway hulls for free in San Diego listed on CL for a while now.

Those would be the best option. as they can float 1000lbs of weight and have built in low aspect keels that will keep the boat well pointed, and will allow for semi-planing thanks to their bulbous hull shape.

Older Hobies tend to squat a lot under power due to all the rocker in the hulls.

This is my converted Getaway:

 

[montero]

You are talented cam operator !

 

[montero]

... and quite spectacular reality there.

 

[gonzo]

Is the motor from the 1930s? They made heavy motors then by today's standards.

 

[philSweet]

@gonzo Today's BF10 manual start long-shaft weighs 98 pounds. Merc Bigfoot with 25" shaft and electric start weighs 125 pounds.

The H-18 spars, sails, rig and lines weigh about the same.

 

[67-LS1]

What is the theoretical hull speed of a Hobie 18? Is a 10hp motor worth the efforts?

 

[SolGato]

What is the theoretical hull speed of a Hobie 18? Is a 10hp motor worth the efforts?

I think a 10hp is about the max I would want, and like I mentioned before a 2-stroke would be advantageous on the older Classic low volume float boats as there is less of a weight penalty.

Of course you’d want manual start with an auxiliary fuel tank you can place further forward.

Getting up into the 15-20hp leg adds quite a bit of weight and is probably not worth it.

There is an older Hobie running around here that has been converted to a power boat that also has a small furling sail probably borrowed from a Tandem Island or something similar.

It does well out in the ocean at speed, but does squat and spray a bit.

I always figured it had a 10hp but never looked closely and will try to identify the motor size the next time I see it.

 

[67-LS1]

Would a 10 power a Hobie 18 much faster than a 2-3? I’m guessing the hull speed of a Hobie 18 is less than 10 mph and after that you’re just wasting fuel. Am I wrong about this? It surely isn’t going to plane.

 

[Squidly-Diddly]

thx, SolGato, plastic indestructible and "de-tuned" Hobie Getaway sounds better for what I've got in mind.