Small Multihull Sailboat Speed Under Power

[surf n siesta]

I'm researching Corsair F-24 trimarans and have read that they can match windspeed up to 12 knts on a reach, but have had a tough time finding info on their speed when motoring.

I found one person saying they cruise at 5knts and another saying they cruise at 6.5knts with an 8hp outboard.

Anyone have any experience using a larger engine on these boats or other multihull sailboat under 30ft?

If they can easily sail at 12knts, then shouldn't they be able to motor at a similar speed?

[StuM]

Originally Posted by surf n siesta

I'm researching Corsair F-24 trimarans and have read that they can match windspeed up to 12 knts on a reach, but have had a tough time finding info on their speed when motoring.

I found one person saying they cruise at 5knts and another saying they cruise at 6.5knts with an 8hp outboard.

Anyone have any experience using a larger engine on these boats or other multihull sailboat under 30ft?

If they can easily sail at 12knts, then shouldn't they be able to motor at a similar speed?

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.

[markpierce]

Small multihull sailboats almost always exceed my speed when there is some wind. Can't remember seeing them under power.

[shaunno71]

I had a Corsair dash 750 - max recorded sailing speed was 20knots. Had a Tohatsu 6hp outboard, motoring cruising speed about 5.5knots. Wouldn’t recommend putting any bigger outboard on it as these boats do not like weight on the transom.

[Chotu]

Originally Posted by surf n siesta

I'm researching Corsair F-24 trimarans and have read that they can match windspeed up to 12 knts on a reach, but have had a tough time finding info on their speed when motoring.

I found one person saying they cruise at 5knts and another saying they cruise at 6.5knts with an 8hp outboard.

Anyone have any experience using a larger engine on these boats or other multihull sailboat under 30ft?

If they can easily sail at 12knts, then shouldn't they be able to motor at a similar speed?

I was confused by this question months ago and had a huge thread on it here.

A64 (and some others) set me straight.

The props are too small in diameter on the outboards to get you to the sailing speeds.

I have a very fast cat and was dismayed that I could only get 8 knots in flat calm out of a 30hp outboard when under sail I’m supposed to be a 20 knot boat.

I thought my pitch was wrong. I didn’t get it. Now I do.

The best analogy is to think in terms of a drag racer. Think of those big fat tires they use to transmit the huge horsepower to the tar and make it go. That huge surface area locks onto the tar and BOOM! That thing is off and flying,

Now replace those big, fat tires with some bicycle tires instead.

What happens?

You have 1000 horsepower, but the tires just slip and you’re going nowhere fast.

That’s exactly what happens to our boats.

In order to make a good power multi that can approach hull speed, you need to use a huge propeller, like a proper powerboat does.

A teeny little outboard prop, no matter how many horsepower you spin it with, just like those bicycle tires on a drag racer, does nothing but slip.

[surf n siesta]

Originally Posted by Chotu

I was confused by this question months ago and had a huge thread on it here.

A64 (and some others) set me straight.

The props are too small in diameter on the outboards to get you to the sailing speeds.

I have a very fast cat and was dismayed that I could only get 8 knots in flat calm out of a 30hp outboard when under sail I’m supposed to be a 20 knot boat.

I thought my pitch was wrong. I didn’t get it. Now I do.

The best analogy is to think in terms of a drag racer. Think of those big fat tires they use to transmit the huge horsepower to the tar and make it go. That huge surface area locks onto the tar and BOOM! That thing is off and flying,

Now replace those big, fat tires with some bicycle tires instead.

What happens?

You have 1000 horsepower, but the tires just slip and you’re going nowhere fast.

That’s exactly what happens to our boats.

In order to make a good power multi that can approach hull speed, you need to use a huge propeller, like a proper powerboat does.

A teeny little outboard prop, no matter how many horsepower you spin it with, just like those bicycle tires on a drag racer, does nothing but slip.

Sorry I didn't find your thread before posting. I had found similar discussions concerning 40-55 foot cruising cats, but wasn't sure if the same limitations would apply on smaller/lighter boats.

So, can you get a larger prop for your 30hp outboard? I'de be curious if you got an extra knot or 2 out of it.

[Augi]

Originally Posted by surf n siesta

Sorry I didn't find your thread before posting. I had found similar discussions concerning 40-55 foot cruising cats, but wasn't sure if the same limitations would apply on smaller/lighter boats.

So, can you get a larger prop for your 30hp outboard? I'de be curious if you got an extra knot or 2 out of it.

If you have a 6 HP outboard on a 24 ft Corsair trimaran and put a much larger prop on it how much will speed increase?

You can tilt the outboard out of the water so the large prop will not effect sailing performance.

[thinwater]

In this case, a Farrier specific answer may help.

When motoring the main (fat) hull is in the water and they don't do much better than monohulls with reasonable horsepower.

When sailing the main hull lifts to where it is barely in the water and the flat sections also allow planing. The float is very slender compared to the main hull, so hull speed does not much apply there.

There is definitely a speed "hump" around 6-7 knots boat speed, when a small increase in wind makes a big difference. You feel it hit another gear as planing starts.

BTW, "matching wind speed" is probably a slight exaggeration unless the bottom is pristine and the boat very light. But 8 knots to weather and 10 knots on a reach in 12 knots is easy. Add the reacher or chute and you can easily excede windspeed; you'll see 14 knots off the wind in 12 knots true, and it can be pushed to about 16 knots, above which it can get weird. 12 knots boat speed can be relaxing.


Most Farrier owners don't motor a lot. They go fast, point high, and are fun to sail. If there is no wind, you stay home, but 5-7 knots is enough to sail. 10-12 is better, 15 is yee-ha!


(I have an F-24)

[surf n siesta]

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.

This seams to be the general consensus, and I understand the extra weight isn't good for boat performance under sail or motor, but promotional content from Corsair will show 4 or 5 people sailing on these boats. Shouldn't I be able to get rid of 2 of those people and replace the 6hp outboard with a 25hp?

The 25hp outboard may not be enough to make much difference in speed, but I'm curious if anyone has tried it. Going 9 knts instead of 5 knts makes a pretty big difference if your destination is 30 miles away.

[Chotu]

Originally Posted by surf n siesta

Sorry I didn't find your thread before posting. I had found similar discussions concerning 40-55 foot cruising cats, but wasn't sure if the same limitations would apply on smaller/lighter boats.

So, can you get a larger prop for your 30hp outboard? I'de be curious if you got an extra knot or 2 out of it.

Here you go, watch me struggle. Ha ha.

https://www.cruisersforum.com/forums...to-232192.html

The anti cavitation plate blocks putting a larger prop on my outboards, so without cutting those off, 8 knots WOT is max.

[thinwater]

Originally Posted by surf n siesta

This seams to be the general consensus, and I understand the extra weight isn't good for boat performance under sail or motor, but promotional content from Corsair will show 4 or 5 people sailing on these boats. Shouldn't I be able to get rid of 2 of those people and replace the 6hp outboard with a 25hp?

The 25hp outboard may not be enough to make much difference in speed, but I'm curious if anyone has tried it. Going 9 knots instead of 5 knts makes a pretty big difference if your destination is 30 miles away.

It really depends on the hull form. My Stiletto 27 would go 12-13 knots with an 18 hp 2-stroke. It came with 8hp and would go 7 knots. The aft sections were flat and it would lift out of the hole. Corsairs have a lot more rocker, are a little heavier, and just dig a deeper hole. The Stiletto was faster off the wind, Corsairs are faster to windward and generally much more forgiving. Compromises.


I wouldn't go past 9.9 hp on an F-24. I actually went down to 4hp because of an engine failure (and I had the 4 hp in the garage), which is enough unless there is wind over ~ 15 knots... but if there is wind the answer is to sail. The 4hp will do about 6 knots in light winds, about 6.8 knots with 6 hp. But I seldom run WOT with the 6 hp.

[surf n siesta]

Thanks for sharing your experience Thinwater.

Would still love to here from anyone else with experience on any F-boat/Corsair. Would like to know outboard horsepower and cruising/max speeds.

I looked at Stilettos, but I think the forgiving nature of the Corsairs will better suit my sailing style/ability.

[Chotu]

Originally Posted by surf n siesta

Thanks for sharing your experience Thinwater.

Would still love to here from anyone else with experience on any F-boat/Corsair. Would like to know outboard horsepower and cruising/max speeds.

I looked at Stilettos, but I think the forgiving nature of the Corsairs will better suit my sailing style/ability.

There’s also this, even though it’s not F-boat specific.

https://www.cruisersforum.com/forums...ng-237846.html

[thinwater]

Originally Posted by surf n siesta

... I looked at Stilettos, but I think the forgiving nature of the Corsairs will better suit my sailing style/ability.

Yes, the Stilettos are best suited to people with an active beach cat background that are comfortable flying a hull in control. You need to bring your A-game when it's gusty.

A friend of mine bought a Prindle 19 this spring. I hadn't sailed a Prindle in 30 years, but the mojo came back fast, and within a few hours I really wanted one! Smokin' fast. It'll tack almost before you get the words out. And then I remembered my age and that the winds were steady that day. But FUN!!


The F-24 is more of a sports car for mature sailers.

[Cadence]

Originally Posted by surf n siesta

I'm researching Corsair F-24 trimarans and have read that they can match windspeed up to 12 knts on a reach, but have had a tough time finding info on their speed when motoring.

I found one person saying they cruise at 5knts and another saying they cruise at 6.5knts with an 8hp outboard.

Anyone have any experience using a larger engine on these boats or other multihull sailboat under 30ft?

If they can easily sail at 12knts, then shouldn't they be able to motor at a similar speed?

Easy answer is no.