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minimum motor sizes


MrWolf

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I ended up with a Honda 15 hp long shaft 4 stroke on Pulse, and its been great for the last nine years. I went Honda partly because we have a good local agent, but also because at the time, and I think still, the Yamaha 9.9 doesn't have a manual pull start as well as a electric start. The idea of taking the top off to 'seagull style' start it after the stereo has drained the battery (cruising) is just too hard.

I've been through a few props and the Honda 4 bladed barge prop is very effective, especially in reverse. And the engine has been very reliable.

Note most props in these situations don't cavitate, but ventilate by air being drawn down into the prop as it pitches close to the surface. Sometimes a larger cavitation plate above the prop can alleviate this problem a great deal.

Also, for better reverse thrust, sometime drilling a few holes (talk to your mechanic first!) above the cavitation plate can let the exhaust out above the prop, rather than the through prop exhaust.

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Back to the original point of the thread.

 

The minimum speed you must be able to go in calm water is :

 

The square root of 3.28 x waterline length.

 

So that is 5.3 knots for an 8.5m

5.7 knots for a 10m

and 7.3 knots for a 16m.

 

under Cat 3:

The 8.5 must carry 21 litres of fuel

the 10m must carry 25 litres

the 16m must carry 40 litres.

 

The 8.5's are pretty sensible cos they they nearly all use 8hp. Which has no problem pushing an 8.5 along in flat water at 5.3 knots.

 

OK but a 5hp would also push the boat at 5.3 knots.

 

What is the point having to carry fuel (21 litres) if you only have a 5hp on an 8.5 ? I mean are you going to feel safe with that. Hooters has a 15hp and motors into a decent breeze.

 

the idea of the thread is not to point out any boats that may be dodgy but rather;

 

The rules as they stand are stupid.

 

Speed in flat water is irrelevant !

 

It is only a matter of time before someone gets in the sh*t if they push this rule to the extreme.

So if we all voluntarily agree to change the minimum motor size to

 

"It can push the boat forward safely into 30 knots in flat water" then

 

A. No boat gets a benefit from having a lighter motor than the next boat (of a similar size)

B. The multihulls become more self reliant.

 

So as not to be hypocritical we have just changed to a High Thrust prop at a whopping price of $126 plus GST. Seems a small price to me for better traction in reverse and at low speed.

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