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minimum motor sizes


MrWolf

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The rules regarding minimum size of motors are more relevant to monoslugs.

 

If your motor can't push your boat into a 20 knot headwind it is too small ! (Let alone 25-30 knots)

 

The only time we really need a motor is when we are in the crap, and then it had better start, and it had better get us to safety.

 

The issue with waterline length being the determining factor of motor size is that realistically the biggest issue pushing a multi into the wind is probably windage.

 

My opinion is that the Multi's should be introducing realistic minimum motor sizes.

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What sort of engine sizes are you suggesting Wolfy, and what does TW have and do you think its enough to bail you out.

I think 8hp on an 8.5 is marginal in a "I really need to motor to windward now" situation given at that stage there are usually waves involved as well.

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This Way up has an 8 hp yamaha, and in a good blow it is marginal.

I would have thought that 8hp would have been a sensible minimum for the 8.5m class.

Afterall, 8hp motors are what most GBEs have been using for the past decade..... some have 15hp for cruising.

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The standard Yamaha 8 with the big foot prop pushed the Voom Ok in 20knots but when there was 25-30 knots and a chop it became a bit hard to make headway, particularly when the boat was loaded up with cruising crap. At that stage you are looking at options other than directly upwind, and at that stage you clearly haven't planned far enough ahead! .

A 10hp probably wouldn't be much more useful - it becomes more of an issue of the prop leaving the water as the wave troughs pass by the centre of the boat so an ultra-mega-super-hyper longshaft 8hp is probably what is required, its just they are a bit hard to come by and possibly a bit hard to store.

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Monoslugs, lighter ones OB powered, of similar size with motors mentioned here seem to do pretty much the same thing.

 

9.9 Hi-trust Yammy (long shaft in this case) or equivalent should very easily sort a 8.5 out even in a big blow with big lumps. Comment made by 9.3mt monoslug that performed very similar to comments made here when fitted (temporarily for a few months) with good 8hp 2-stroke. The difference between that and the 9.9 is like night and day, as is the fuel bill, the 2-stroke was like a drain.

 

There is a 15kg odd weight difference though. Std 2 stroke 8 compared to a 9.9 4-stroke with lectric start. Loose the lectric start and save around 5-6kg I'm told.

 

I would think if the 8.5's can keep the prop in the water a 8hp, with the big prop, should be OK 95% of the time. Sounds like that is more the niggle than the motor size. I remember we had fun at some motoring times on our GBE.

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My SR26 had an 8 hp outboard with a long shaft and boat prop. The one really scary time was trying to get home through Sargent Channel and the boat just wouldn't make headway. It was a short steep big chop. The outboard kept popping out of the water when the boat rolled. Ok rolling isn't exactly a raft problem, and if you have then the size of the outboard isn't an issue anymore.

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From my expierience an 8hp high thrust is fine for an 8.5 as long as its high thrust/long shaft and the prop is in the water.

 

the biggest issue with the Cats iv sailed is the prop cavitating, you could have a 25hp and it would still go backwards in 30 knots!!!!

 

I wouldnt be prepared to upgrade from my bigfoot yami 8hp, its heavy enough to load onto a dinghy and take ashore for servicing as it is.

 

However as Tim has said its the windage which is the problem and sensible rules would benifit everybody, the difference in windage between 5.5m wide 8.5m cat with 12.6m airdraft and a slightly bigger wider cat with taller rig would be huge?

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What sort of engine sizes are you suggesting Wolfy, and what does TW have and do you think its enough to bail you out.

I think 8hp on an 8.5 is marginal in a "I really need to motor to windward now" situation given at that stage there are usually waves involved as well.

 

The only "test" the size of motor needs to pass is whether you can motor into say 30 knots, even in a relatively flat sea.

So therefore if a boat had just proven beyond doubt that it can't motor into 20 knots then its motor would be too small.

 

In our case we have an 8hp Mercury, but not with a High Thrust on it. Fortunately with the well in the middle it never lifts out.

We got back to the piles after a rum race one night (the race Thunder ended up on the seawall)in over 40 knots so i guess we aren't too far off with what we have.

 

Really though if everyone has sensible motors (in other words no one else makes a weight saving by having one that is lighter) then its to everyones benefit.

For example if we had a multi rule for example 8.5's have minimum 9.9 hp, the 9m boats a minimum 15hp etc.

As long as everyone is "overpowered" as it were, we are all then more capable of getting ourselves out of the crap and we wouldn't need to rely on the Coastguard to tow us home.

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I wouldnt be prepared to upgrade from my bigfoot yami 8hp, its heavy enough to load onto a dinghy and take ashore for servicing as it is.

Out of interest, is that one a 4 stroke hi-thrust, just a 4 stroke or a 2 stroke?

 

Good pre-emptive thinking going on here by the way guys. Nice call Mr Wolf.

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I personally dont believe that a 15hp is necessay on a 9m race boat. For cruising it definitly would be great, but for racing under cat "4" it is definitly over kill. Maybe more relavent for cat 3 when you have to be more self sufficient and are likely to be on the water for longer periods when you could get caught out in nasty weather.

 

Although I said in a good blow that our 8hp was marginal, it comes down to how you handle the boat as much as how powerfull your outboard is, and I wouldn't be rushing out to upgrade.

 

out of curiosity, how many multihulls have got into trouble in the past year due to insufficient motor size ? In most cases it it the motor not going at all that gets people into trouble..... in 15 knots of wind........ maybe we should introduce outboard warrent of fittnesses for the multis as well...... not.

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I wouldnt be prepared to upgrade from my bigfoot yami 8hp, its heavy enough to load onto a dinghy and take ashore for servicing as it is.

Out of interest, is that one a 4 stroke hi-thrust, just a 4 stroke or a 2 stroke?

 

its a high thrust 2 stroke about 27 kg.

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The reason the Yamaha 9.9 performs well pushing cats around is not to do with the hosrepower, but the gearbox ratio. Its the only motor on the market (that I know of) that has a 3:1 drive ratio. The Yamaha 15hp, and all others (again that I know of), have a 2:1 ratio.

Outboards rev at about 4500-5500 rpm maximum. What this means is that the 9.9 swings a big prop slowly, eg 1500rpm, whereas the 15hp outboard swings the prop at 2500 rpm. Efficiently driving boats at displacement speeds is about big props going slowly, the Yamaha 9.9 (and ancient Seagull outboards) do this. When using 2 x 9.9 (not an option on a race boat I know) you get a lot of prop area, far more than 1 x larger hp engine.

Since the cat does about the same speed (displacement speed 6-9Knts) and most outboards deliver their power at high rpm only, there is a lot of slippage and other problems when using a single 15hp+ engines. You may go a bit better in ideal conditions but when you have to push into 30knts, the 1 x Larger hp engine's prop will just cavitate under the strain, the 1 or 2 x 9.9's will move relentlessly forward.

 

...The Yam 9.9 produces 9.9Hp at 4.5k revs against 5.5k for most other engines this size, has a 2.92:1 gearbox to an 11 ¾” x 9 ¼” prop with a large surface area. The gear ratio/prop is optimised for 7 – 13 knots boat speed.

As a comparison the Honda 9.9 (at 5.5k revs), has a 2.33:1 box with a 9 ¼” x 8” prop with small surface area. The Honda 8 has a 2.4:1 box with the same prop. I've never found any literature on what speed they're optimised for. They do have the longest shaft lengths though.

The Yam 15 (at 5.5k), has a 2.08:1 gearbox and a 9 ½” x 9” prop optimised for 13 – 20 knots.

The companies change these specs a bit each year but nothing comes near the Yam 9.9 4 stroke for pushing a yacht.

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The Yamaha 9.9 and 15 are the same engine they just have a different gearb ox ratio to give the hourse power which effectively reduces the torque available under load.

 

we out a 15hp in the shaw 9m as its the same weight so why take the extra hourse power we have a barge prop on so high power and we motor at about 9knots and have no problem motoring into a breeze which is what you want we stripped a heap of gear off the motor saved about 5-6 kgs by getting rid of the tilt bracket etc as ours is in a well and goes up and down.

 

Cheers

 

Gappy

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re motor sizes I have a 8.5 Given Gulf Tiger here in Opua,we had a 8hp which was just not doing any good,we have put a 9.9 Yamaha long shaft electric start with barge prop its great.We have come up the channel when the waves have been up & over the outer wall of the Opua marin due to the fetch & we have had to punch into it to get around to the mooring at the end of the river no problems at all.Fuel is good Opua to Brampton Reef & back twice used 5 litres in total with 6 up & all the gear the girls needed. Great motor.

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Some good info EZIFOLD thanks.On our GBE we have a 9.9 sail-drive yamy and although quite heavy it does the job well.It uses about 1L an hr at 6knts. If we were racing other GBEs the weight would be a disadvantage,in our situation no need to change.I have pushed into a good chop and 28-30 knots going up the harbor at 5 knots and it was the one time we had no reserve. While getting our motor serviced they lent me a 3hp short-shaft ,the boat floated higher and went notably faster that night sailing

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All good gforce

We ran a Yamy 15hp on a 28' lead swinger (pig) out of Nelson, the prop even at the bottom of the extra long shaft was an egg beater at best when ever we had to push into wind against tide to get back through the cut or up around D’Urville Island.

Changed to a Yamy 9.9hp high thrust and observed a definite (under statement) advantage.

Weight was not really of concern as the lump of lead outweighed any motor weight disadvantage.

For the race oriented multi’s on here perhaps as mentioned prior post by someone do some strip out of the non-essential.

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i would have to agree that the big prop yamaha 9.9 is the best for the job and as long as you can keep the prop in the water it should be more than enough for an 8.5 m multi. I had 2 on a 46 ft cat and they would do up to 9 knots in calm conditions or 5-6 cruising on one motor, and these were 20 + year old motors. There was a good article in the multihull world magazine about enlarging the cavitation plate and redirecting exhaust gases to improve performance and reduce cavitation. I have heard of people changing to the 20 hp honda at the same weight and getting around 12 knots from a 36 ft cruising cat with 2 motors, they were pretty happy with the hondas tho they are a fair bit more money than the 9.9 yammy when buying new. I would think that a gbe or similiar with a single 9.9 Yamaha hi-thrust would do 9 knots pretty easy and as long as you can keep the prop down make headway against 30-40 knots?

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I may be wrong but I believe that Sundreaner had one 15hp in it when launched Sd may correct me on this but the Hondas he has put in I think 25's in both hulls give you plenty of grunt especially when we were doing winters series in big breeze trying to get back on the marina at westhaven you were under seemi control I can;t believe how many boats are under powered. The Andar the open 50 I think had a 18hp diesel in it and it just went to where if it was breezy they have repowered it now also its just not worth it safety has to be a factor not just the weight

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