Under-40' Cats with Outboards . . . Why Not More ?

[Jimbo485]

That is very useful info to know, 44ccat! Thanks, mate!

[Jimbo485]

I am looking on the net and I can find a 20HP long shaft (20") but not 25" nor 28". Should be called BFP20DKXRT, but google shows nothing. In the last few hours, I have managed to partially decode the Honda system.
3rd letter is power thrust
4th and 5th digits refer to HP
DK is still unknown
8th letter can be S, L or X and refers to short, long or extra long shaft
9th letter is R or H and refers to handle controls or remote controls
10th letter is T and still unknown.

[boatsmith]

Sorry I missed the Honda part of that power head swap. Yamaha to Yamaha OK. The Honda hi thrust is only available in the States in a 25" shaft as a 15hp. I have heard that in OZ that the 20 does come with an extra long shaft. It is the same weight as the 15. I believe the only difference is the carb and the RPMs.

[Jimbo485]

So I should be able to buy the 15 HP Honda with the 25" shaft via the US, get it sent down here, order a new carb for the 20 HP thru my father in Australia, get him to courier it over here and then swap out the carbs? And finish up with a 20 HP with a 25" shaft?!?!?!

[boatsmith]

From my info that would be correct....but before I dropped money on that program I wuld have a talk with the Honda tech people.

[harlie]

It's more than just the carb. Check valves & manifolds

[Therapy]

Originally Posted by harlie

It's more than just the carb. Check valves & manifolds

Beat me to it.

I think "Honda" knows it can't be done cause 20s cost more than 15s. They aren't going to put up with it being done.

Many years ago I was going to change my 9.9 Yamaha to a 15. Same motor right? Just add a carb!! NOT! Reed valve size and manifold cutout different size. That meant a different block.

[kaimusailing]

My boat was an unfinished project and a lot of stuff was self designed by the previous two builders. They opted for a single outboard to drive a 44' catamaran. The second builder custom made a "sled" to mount the motor, and he spec'd a 50 HP Yamaha 4 stroke High Thrust, so the sled was designed just for that motor. The sled is like a large aluminum vee bottomed tray with a transom. It hinges at its forward edge. It can be raised to be flush with the bottom of the bridge deck and the outboard motor has power tilt so that there is nothing in the water when it is all raised. This motor drove the boat over a 1000 miles from New England to Florida and was very reliable. Later I had problems with carbs fouled due to ethanol gas and it is a continuing problem. The HT engine can have an efficient prop to drive at normal boat speeds, up to about 10.5 knots. The local marina used the same engine on their water taxi, but have changed it. Perhaps they had the same carburettor problems. Now they are fuel injected and that problem is solved. Other boats like my design have used other engine installations, including "long tail" with diesel mounted on the bridge deck, single diesel in one hull, or twin diesels. The weight of diesels is not a problem in the larger cats and twin engines offers redundancy and all the other features mentioned by previous posts. I have had the ventilation problem when powering into a steep chop. My speed at that time was reduced to 2.2 knots. Other times in normal conditions I've found the 50 HP pushes me faster than other good sized boats, except when the carbs clog up and I have to ask for a tow.

[double exposure]

I've read some good,but quite a bit of bad info on this forum, especially those who say you can't put outboards on a 35+ cat, that they burn to much fuel, that they don't have good charging abilities. My boat is 38', 40'LOA w/ the twin Evinrude 115 E Tec outboards, fuel burn rate is better than a trawler at 8kts, and they have 50amp alternators on each engine. Oh yeah, and when there is no wind, I can do 20 kts all day long.
To all those people, my cat can beat up your cat.

Sailing Catamaran Double Exposure, 0 to 20 knots in less than 25 seconds on a 40' cruising cat - YouTube

[bill good]

Why did you not let the wind generator charge was well!!

Nice

Regards Bill

[double exposure]

I've also got (2) 50 watt solar panels and 100 amp battery charger on the gen/inverter setup. So along w/ the 50 amp alts on the engine and the wind generator, charging batteries is the least of my worries.

[bill good]

Just joking!! How much advantage would you get with the sails up & using the apparent wind at 20Kts??

Bill

[geoleo]

OK Guys -heres my 36ft Woods Catamaran with its 15HP Mercury/Yamaha with a 25 inch extension (kit Made in Ohio) on its very custom all aluminum bolted up/down bracket which gets the cavitation plate a full 9 inches under at rest......only took a year to develope.

[Therapy]

Originally Posted by double exposure

I've read some good,but quite a bit of bad info on this forum, especially those who say you can't put outboards on a 35+ cat, that they burn to much fuel, that they don't have good charging abilities. My boat is 38', 40'LOA w/ the twin Evinrude 115 E Tec outboards, fuel burn rate is better than a trawler at 8kts, and they have 50amp alternators on each engine. Oh yeah, and when there is no wind, I can do 20 kts all day long.
To all those people, my cat can beat up your cat.

Awesome dude!!

[Jimbo485]

I assisted some mates take their boat thru the Panama Canal yesterday and today. It was interesting to participate and watch when we rafted 3 boats for the last 3 chambers. There was a heavy displacement, 40+' mono to port, a 42' light weight Crowther in the middle and a 28' mono to starboard. The Crowther in the middle was in control of the raft while entering and leaving each chamber.

What did the Crowther have in terms of propulsion?
Twin 9.9 HP outboards.
Worked like a charm, no dramas.

The monos has inboard diesels.