Prop Size: 17" vs 18"

[cat man do]

OK, so that has me covered with an 18 or 19 inch fan and 3 inch clearance (for the 19)

Too late now but are there any advantages with having more clearance like in my above photo?

A clear Disadvantage was more draft

Dave

[Bob the builder]

Hi all I am anew member,and I need more motoring speed from my Fusion 40.Motors are 30hp yanmars with gori 3blade folding. If I double the HP how much extra speed could I expect?Fuel consumption not a major consideration.Increase in weight not huge as going to Volvo D2 55hp or75hp.Thanks for any help.

[Evan]

How fast are you going with the Yanmars? About 8-8.5 knots full throttle?

Increasing from 60 total to 110 HP may increase your speed a few knots. You will need bigger props though - do you have a saildrive which may limit this (and would it accept the increased HP?)

If you have a straight shaft, you may need to increase shaft diameter and of course also prop size. You may run into a clearance issue (shoot for 15% propeller diameter as the minimum clearance between prop tip and hull above)

[Karletto]

there are books about propeller that can hold your door in the room while the wind is blowing through your apartment. it depends on RPMs, shaft/ keel/ hull measures and shapes; a lot of formulas

with this info only i suggest bigger propeller. probably an expensive propeller is much more developed too.

[Alan Wheeler]

Too late now but are there any advantages with having more clearance like in my above photo?

The bigger the distance the less affect of turbulence from other structures. The cleaner the flow, the better the performance and the less noise.

Increasing from 60 total to 110 HP may increase your speed a few knots. You will need bigger props though

A very silly thing to say. How can you determine that more Hp will give you more speed when you have no idea of WLL for a starter. Going from 60Hp to 110Hp could mostly likely result in no more speed but an enormous fuel increase and a hell of lot of heartache.

with this info only i suggest bigger propeller.

With the info given you can not suggest anything. The information and wisdom you gained from your book should tell you that.

[eyschulman]

Just one point the rule of thumb about getting to within 10% of rated HP rpm may not be so good for the newer lighter turbo charged motors. A very large # of sailboats are running all over the world with some serious over propping and not so many blown motors. However with the newer gutless motors watch out- prop to rated rpm or slightly better or that motor may not last as long as the older heavy iron.

[cat man do]

Originally Posted by Alan Wheeler

The bigger the distance the less affect of turbulence from other structures. The cleaner the flow, the better the performance and the less noise.


.

Thanks for that

Compared to a lot of powered and sail vessels the underwater lines are very clean and everything is a foil, so I would hope turbulence is down

I am also hoping noise may be taken care of somewhat by a healthy layer of foam as a core which is in addition to the 16mm timber core of the main part of the vessel.

That and the prop is 7 metres from living space.

The most important factor for this design was at the end of the day I needed 850mm MAX draft so as to still get into some of my secret spots discovered with my last cat.

Anchoring out is not an option due to depth (50m +)

Dave

[cat man do]

Originally Posted by eyschulman

Just one point the rule of thumb about getting to within 10% of rated HP rpm may not be so good for the newer lighter turbo charged motors. A very large # of sailboats are running all over the world with some serious over propping and not so many blown motors. However with the newer gutless motors watch out- prop to rated rpm or slightly better or that motor may not last as long as the older heavy iron.

Exactly why I chose my slow spinning 3.3 liter naturally aspirated cummins's that put out 65hp and an insane amount of torque.

They are about 55kg/side heavier than the turbo yanmar, but thats = to one person on a 50 ft cat that only has 2 people on it anyway.

Dave

[David M]

During my last engine installation, Cummins would not warrant the engines if they could not reach full rated RPM. I'm sure they had good reason for that.

Also, once you reach hull speed, you had better have a planing or a semi-displacement hull otherwise all you are doing is pushing a wall of water and not really going any faster. Therefore its pointless to purchase an engine whose horsepower is greater than the horsepower it takes to reach hull speed for a full displacement boat.

[Whimsical]

Originally Posted by Alan Wheeler

A very silly thing to say. How can you determine that more Hp will give you more speed when you have no idea of WLL for a starter. Going from 60Hp to 110Hp could mostly likely result in no more speed but an enormous fuel increase and a hell of lot of heartache.

Allan
He was responding to the enquiry about a Fusion 40 which would hae a WLL of 36 or 37 feet so a hull speed of about 8 Knts. It is my understanding that while a cat has the familiar speed hump at hull speed it is actually a lot flatter curve and more power will in fact give an increase in speed. So the extra few knots would probably be feasable with double the power. IMHO pointless but feasable.

Mike

[Davey1000]

Originally Posted by Badsanta

I found a prop calculator on boatdiesel.com that seems to work well.

There is also a good one by Vicprop of Canada. Castle Marine have quite a good one if your computer can run 16 bit software. Unfortunately this is incompatible with Windows 7 sixty four bit. Hope no one is offended by the link. It looks to be a good calculator as it asks about the boat. Some calculators only take four inputs to calculate the fifth but more data is needed.

Vicprop - Prop calculator for Displacement and semi-displacement hulls