More Speed!

Our 40foot sailing catamaran has twin 30hp in board diesels, and motors far too slowly, I was just wondering what your opinions are on the installation of 75hp Volvo saildrives which are only slightly heavier than the yanmars. What increase in speed could i expect? I've been told that the boat would "bury its bum" under under power. Current motoring speed is 6-8knots. Boat wieghs approx. 6 ton, and is 2 years old.

 

Have you checked both motors are in gear!!!

Sounds slow for a 6t 40ft cat with 60HP.

Is it 6 knots or 8 knots? There is a big difference. Could understand such a wide range if you are going into a howling gale and then going with it or for and against a current.

I would be looking to see what is draging. How many tonne of marine life are you carrying with you? Are the props clean?

Are the motors going to full rpm? Meaning are the props under pitched. What size props?

Do some proper testing in an area without current and low breeze or with and against the current using a GPS for speed determination. You then have a baseline for improvements.

What is you fuel rate at full or near full throttle - does it agree with diesel data?

Depending on hull shape you would expect a cat to respond well to extra power. Do have any detail on the hulls? What design?

Something does not compute but you have proposed a vague question without much information to go on.

Does your waterline sit correctly for the guesstimated displacement? What has been added to the boat?

Hope this gives you somewhere to start.

Rick W.



Rick W.

 

re

Thanks Rick,
Answering your questions,

6 knots sometimes more sometimes less depending on conditions

Hull etc regularly cleaned running gori 3 blade folding props

It doesnt do too badly as far as agreeing with the data provided, little over but no more than expected

The boat is a Gary Lidgard designed fusion 40 manufactured in Arlie Beach

The boat sits well compared to the waterline, even when fully loaded with full passengers sits on the factory waterline if not above.

I was considering changing boats, but this is such a great boat, accom, sailing, looks etc. I'm prepared to put the dollars in to getting the bigger motors if we get the desired results, she sails well regularly reaching 15 knots. Today with only the screecher up with 15 - 20knot winds averaging a boat speed of 8 - 10.5 knots.

Limited time means we usually have to motor ro home to go to work after a weekend away.

Bob

 

I did some calculations on a 40ft cat displacing 6 tonne to give you an idea of what to expect. It indicates why I think something is amiss.

The power is based on overall efficiency of 70%. Good props will get over 80% so there is some allowance for appendage drag and windage but not a huge amount.

This is what you could expect with everything nice and clean, good lines, boards up, no odd long keel, low wind, mild sea state etc.

You should be easily pushing over 10kts. Hence there seems to be something wrong. Better to understand what is wrong before you go doubling the power.

Rick W.

 

sometimes, these cats which sail at high speeds, just stuff the clutches of the yanmar, because they overun the speed if youa re motorsailing
Do the motors sound like they are not loaded?
tie the boat to the jetty and run against your springs, then do your engines load to full load rpm, or do they sound like they are on high Idle?
Oh and 75 shp, is somewhat of a huge overkill

 

The fact that you are getting over 10kts under sail would indicate the motors are not delivering power or not applying it efficiently. 60HP is a lot of power for a 6t cat.

What diameter are the props? Is there gearing to the prop? What engine rpm for peak power? What pitch are the props?

I suspect the gearing is set up for pushing into a good breeze so you are not getting the best in calmer conditions.

Gori props have a good reputation but I have not handled one so no idea what problems they might suffer.

Are you certain the blades are unfolding properly - not binding up.

The fusion site indicates top motoring speed of 9.5 knots. I would expect a bit better with the right props but it is a hell of a lot better than 6 knots.

Rick W.

 

LJ
Would this still occur if the props fold correctly?


Rick W.

 

By the way that is a nice boat. You gotta be able to do better than 6 knots with two 30HP sail drives.

Rick W.

 

hi rick thanks for your interest. maybe its the props, any idea how to source alternatives. ifind in the marine industry people tend to sell you what they have or what is most profitable for them not what is the best for you...but i guess thats life..cheers jester

 

Bob
You need to get a good handle on what you have. Try to work out what is going on. Compare it with same type of boat but with different drive units and different props.

If you can provide the prop, motor rpm and gearing detail I can determine what performance you should expect from the combination. My first thought is that the props are not selected well for the boat. More like they were intended for a single motor in a 30 - 35ft monohull and you are just running into limit on rpm.

My own experience with a folding prop was on a Bukh and I was pleased with the way it performed.

There is a lot of benefit with a folding prop on a nice sailing boat so I would be loathe to suggest changing to a fixed blade. A large diameter high aspect two bladed prop will give good motoring efficiency and not absolutely kill sailing performance though.

Best to do more calculations and comparisons with other similar cats before you go making expensive changes. Get the motor and prop data first.

Rick W.

 

hi rick prop is gori 3blade folding 16.5x12. engine is yanmar 3ym30 full engine speed is 3600rpm.gearbox is sd20 ratio is 2.64.appreciate your assistance...jester

 

Bob
The Spec for the motor gives peak power at 3200 rpm. This translates to 1212rpm at the prop. Under these conditions the prop will absorb a maximum of about 27HP at 4kts. On the other hand You should be able to get around 10kts with each engine working at 7HP.

The props are not going to allow you to deliver full HP up around the top speed of the boat. You need more pitch. If you can go up in diameter it will help as well although the efficiency is not too bad. With existing prop efficiency would be around 70% at full speed.

If you went to 14" pitch you could expect to get the 10kts and still not bog the engines under heavy conditions.

If you shop around for props then let me know what you can get easily I will give you some estimates.

You could get some useful data by just running on one engine. You should find that you do not lose too much speed because the prop will load up quite quickly as speed reduces. Do this with both motors alternatively to confirm they are both doing their job. Also the motor should not be labouring. They should still be able to get to full rpm without black smoke. This verifies that they are just spinning out.

Rick W.