Small Multihull Sailboat Speed Under Power

[MultiCountryDon]

F-24 MK II sold in Jan 2020
Best under sail during 4 years of ownership 21.3 kits
Scary single handed
16 kts-wonderfull
6 hp outboard best 6.3 kts WOT

Don

[Cadence]

Originally Posted by MultiCountryDon

F-24 MK II sold in Jan 2020
Best under sail during 4 years of ownership 21.3 kits
Scary single handed
16 kts-wonderfull
6 hp outboard best 6.3 kts WOT

Don

Sounds very realistic. I've had two tris.

[mikedefieslife]

Originally Posted by StuM

Nope, an engine big enough to provide the drive that the sails do would be heavy enough to weigh the boat down and destroy its sailing performance.

Would it? An American weighs 100kg, that gets you a powerful outboard. Ok they are movable ballast, but still, the boats are probably specified for 4 people.

If you're European you are limited to 75kg-80kg.

Eitherway a 25hp motor if 4 stroke, or a 50hp 2 stroke.

It will never be as fast as sailing (due to hulls shape etc..), but should be a fair bit quicker than 6.5knts depending on where you can mount it.

[alansmith]

Surf, my Condor 30 runs at 7.5 knots with my 9.9 tohatsu. I reproped it. Condor is a heavier, beefier and more comfortable trimaran than a Farrier/Corsair.

I don’t know where you got the notion that minimalist trimarans should have an engine large enough to equal sailing speeds. It is counterintuitive. You don’t seem to understand the design yet.

Minimalistic tris are built for speed under sail. We are sailing in 5 knots of speed when everyone else is motoring. If you put a 25 hp 4 stroke engine on your stern she will squat a little bit. You only need a motor for steerage and get home if the wind dies or something breaks. You should be thinking somewhere between 4-6 ho for a boat that small. The more weight you carry on a small trimaran the worse she will be sailing. You don’t buy a trimaran to motor fast. You buy a trimaran for performance sailing and portability.

Another thing to say. The F-24 are rather cheaply built. If you sling a 25 hp on the stern and haul her over any distance you will have to dismount the motor From the transom. That transom is not built to withstand the stresses of a 150 lb motor. I’m sure you could beef her up. Have fun trying to lay glass and resin back there from the inside.

I’m not trying to criticize you. I’m trying to get you to understand what tris are about. You also have to understand that larger engines are more fuel thirsty than a 6 hp. Which means you have to carry more fuel. Where to store that extra fuel? Have you been on a F 24? They are freaking small. Wish you luck. I would highly recommend that you go for a used F27 or F 28 or even a used F31. Can live on one of those for a month easily. F24????

[Chotu]

Originally Posted by alansmith

Surf, my Condor 30 runs at 7.5 knots with my 9.9 tohatsu. I reproped it. Condor is a heavier, beefier and more comfortable trimaran than a Farrier/Corsair.

I don’t know where you got the notion that minimalist trimarans should have an engine large enough to equal sailing speeds. It is counterintuitive. You don’t seem to understand the design yet.

Minimalistic tris are built for speed under sail. We are sailing in 5 knots of speed when everyone else is motoring. If you put a 25 hp 4 stroke engine on your stern she will squat a little bit. You only need a motor for steerage and get home if the wind dies or something breaks. You should be thinking somewhere between 4-6 ho for a boat that small. The more weight you carry on a small trimaran the worse she will be sailing. You don’t buy a trimaran to motor fast. You buy a trimaran for performance sailing and portability.

Another thing to say. The F-24 are rather cheaply built. If you sling a 25 hp on the stern and haul her over any distance you will have to dismount the motor From the transom. That transom is not built to withstand the stresses of a 150 lb motor. I’m sure you could beef her up. Have fun trying to lay glass and resin back there from the inside.

I’m not trying to criticize you. I’m trying to get you to understand what tris are about. You also have to understand that larger engines are more fuel thirsty than a 6 hp. Which means you have to carry more fuel. Where to store that extra fuel? Have you been on a F 24? They are freaking small. Wish you luck. I would highly recommend that you go for a used F27 or F 28 or even a used F31. Can live on one of those for a month easily. F24????

Coup you please tell us how you repropped it?


What pitch and diameter did you start with? What was the top speed?

What pitch and diameter did you end with? What was the improvement top speed?

[surf n siesta]

Originally Posted by alansmith

Surf, my Condor 30 runs at 7.5 knots with my 9.9 tohatsu. I reproped it. Condor is a heavier, beefier and more comfortable trimaran than a Farrier/Corsair.

I don’t know where you got the notion that minimalist trimarans should have an engine large enough to equal sailing speeds. It is counterintuitive. You don’t seem to understand the design yet.

Minimalistic tris are built for speed under sail. We are sailing in 5 knots of speed when everyone else is motoring. If you put a 25 hp 4 stroke engine on your stern she will squat a little bit. You only need a motor for steerage and get home if the wind dies or something breaks. You should be thinking somewhere between 4-6 ho for a boat that small. The more weight you carry on a small trimaran the worse she will be sailing. You don’t buy a trimaran to motor fast. You buy a trimaran for performance sailing and portability.

Another thing to say. The F-24 are rather cheaply built. If you sling a 25 hp on the stern and haul her over any distance you will have to dismount the motor From the transom. That transom is not built to withstand the stresses of a 150 lb motor. I’m sure you could beef her up. Have fun trying to lay glass and resin back there from the inside.

I’m not trying to criticize you. I’m trying to get you to understand what tris are about. You also have to understand that larger engines are more fuel thirsty than a 6 hp. Which means you have to carry more fuel. Where to store that extra fuel? Have you been on a F 24? They are freaking small. Wish you luck. I would highly recommend that you go for a used F27 or F 28 or even a used F31. Can live on one of those for a month easily. F24????

I'd be happy to get an F28 instead, but they are substantially more expensive from what I've seen. I would for a 27 over the 24, but Im not a fan of the aft cabin layout. Either way I'm just trying to understand the boat's abilities at this point. I'de be happy to hear from anyone who has any F-boat, or similar that can tell me their outboard Hp and cruising/max speeds.

I understand most people don't seem to use these boats to go places. They seem to be used mostly by racers and day sailers that just want to sail nowhere fast and therefore prefer the lighter outboards that they only use to get in and out of their slip. I am trying to find a boat that I could use to get from anchorage to surf spots BEFORE the wind picks up(motoring) then sail home or on to the next location. I am attracted to the F-boats for their sailing characteristics and their trailer ability like everyone else. I won't have a harbor, so the shallow draft and light weight will be key to pulling it out on the beach.

I'm pretty sold on the F-boats for my needs/wants. I'm just trying to learn as much as I can in an attempt to minimize my compromises.

[surf n siesta]

This is what I have found so far:

F-24
Hp = Knts/WOT
4 = 6
6 = 6.3
6. = 6.8
8. = 8.5
15 = 9.3

F-27
6. = 6.5

[mikedefieslife]

Originally Posted by surf n siesta

This is what I have found so far:

F-24
Hp = Knts/WOT
4 = 6
6 = 6.3
6. = 6.8
8. = 8.5
15 = 9.3

F-27
6. = 6.5

8hp has to the sweet spot there given that a Yamaha 8hp and Tohatsu 9.8hp are 26kg. (2 stroke). Going up to 15hp the Yamaha is the lightest at 36kg.

[tkeithlu]

That 6.5 knots is probably hull speed. Hull speed for a 23 ft LWL monohull is 6.4 knots. Yes you can hang a big engine on it, thereby mucking up your balance and sailing performance, and yes, by pumping in enough power and using up gobs of gasoline, go faster under power. You would be trashing a beautifully designed boat. Why would you want to do that?

[mikedefieslife]

Originally Posted by tkeithlu

That 6.5 knots is probably hull speed. Hull speed for a 23 ft LWL monohull is 6.4 knots. Yes you can hang a big engine on it, thereby mucking up your balance and sailing performance, and yes, by pumping in enough power and using up gobs of gasoline, go faster under power. You would be trashing a beautifully designed boat. Why would you want to do that?

An 8 or 9.9hp 2 stroke weighs the same as a 6hp (which it takes to 6.5knts) anyway, so he might as well have the higher powered engine.

[surf n siesta]

Tkeithlu, I tried to explain my desire to motor at a higher speed earlier. I want to use the boat for transpiration as well as just fun sailing. Of course I will sail when the winds are favorable, but it would be nice to motor quickly when I need to motor.

I plan to get an F-boat(or similar) when I finish building my garage down in Baja, Mexico, so the 2 stroke Yamaha Enduro is the outboard I would gravitate to before doing any research on outboards for sailboats.

Yamaha Enduro 2 strokes, according to the Yamaha Global website weigh in dry at:
8=29kg/64lbs
9.9=40kg/88lbs
15=41kg/90lbs

Would the extra 26 lbs really destroy the sailing performance of these boats?
I may need to go on a diet...

[mikedefieslife]

The Yahama 8hp 2 stroke I was talking about is the 'normal' one not the Enduro one. The Enduro 8hp is supposed to be a poor engine.

Again with the 15hp from Yamaha. The 'normal' version is lighter at 36kg, or 38kg for long shaft, and is more refined (according to users) than the Enduro model. The 9.9 Yamaha uses the same block and everything at the 15hp model, so it makes not sense.

The Tohatsu 9.8hp outboard is 26kg, possibly 28kg in long shaft form. It's not as popular at the 15hp yamaha Enduro, but is still possibly the 2nd most common engine in the Caribbean, and a good engine.

I believe that high thrust props are available for all of these models.

[smj]

The Telstar 28 trimaran will motor at 6kts with a 9.9hp outboard or 13-14 kts with a 40hp outboard.

[purvisgs]

Lots of factors in play here. I don't think you will get the answer you are looking for, which seems to be pretty corsair (f-24) specific. Because no one has done it. The folks who own one cover distance on the trailer and with the sails up. On these boats, since the early 2000's, the 6hp tohatu sailpro is often chosen due to weight (~60lb). The boat, motor mount & transom isn't built to hold more than the next size up, (9.8hp tohatsu ~81+lbs or older Yamaha 9.9 ht ~100+lb). If you joined or searched the f-boat group, I'd imagine you might find info on top speed with 9.8hp. It would be a relatively small gain over the 6hp...



The tohatsu "high thrust" (four blade/sailpro) prop -mentioned by others earlier - available for 6hp or 9.8/9.9- is a slight improvement, but a real "high thrust" (gearbox/lower unit) with different gearing and a slower moving (slightly) larger prop (such as the older yamaha 9.9 4stroke) is probably more useful than additional hp.



Above somewhere around 6-7 knots or ~8hp or so, as others have said, it takes a lot more hp and fuel for diminishing top speed gain.



I'm not suggesting it's a good idea but the next options up weight wise are Suzuki/Tohatsu/Yamaha 20hp (100+lb), or Yamaha 25 high thrust (141/154+lb). I don't think anyone who has sailed a f-boat very much would be realistically looking to put one of these monsters on the back.



There are also 2 stroke options that provide good power to weight but will burn more fuel.



For the f-24 - the transom, outboard bracket/mount would have to be modified and heavily reinforced, or a custom side hull outboard sled built & designed, which is all pretty counterintuitive if you're a sailor at all.. I suspect once you spend some time on the boat in question, you would agree.



Look at the catamarans around 30-36ft that people have sometimes put x1 or x2 20-50hp outboards on, typically heavier w/round hull shapes. But sometime light displacement too. They go a little faster but I don't think you often see any motoring around flat out, maybe 8-10 knots or so, though, and they also have more hull length to work with. It's possible they can go a little faster, but they don't (usually- it seems). Corsair/farrier are light and responsive. You absolutely don't want any additional weight hanging off the back.

[JohnEd]

I see this is quite an old post but did you ever go ahead and fit a much larger o/b to your Tri. ?
I am thinking about fitting a 25hp 2 stroke to my Corsair Sprint. I already have a 9.8hp *4 stroke on the boat. This will push the boat at 11 knots max power in a calm sea but 7 knots is a more realistic cruising speed.
Not sure what effect this has made on sail performance but I have clocked 15 knots on a reach with 3 big adults on board even though I reduced the main sail/raised the boom to accommodate a bimini .This was with the screecher up which I use all the time in our light wind area. So under sail the boat is fast enough for me especially as I don't race it.
Anyway, my best guess is that a 25 hp, Tohatsu 2 stroke which only weighs 18 kgs more than my 9.8 four stroke* "might '' increase speed by another 5-6 knots and I would be happy with a 12-14 knot cruising speed in a calm sea.
I know some Sprint sailors will be horrified at the idea but when the wind dies try motoring for half a day at 4-5 knots with a 5hp* in 100F , 1000 km for the equator.
Finally I should mention that the transom was strengthened 2 years ago (no problems so far) and I would strengthen*further if I went ahead with the 25 hp idea..