jet drive v/s prop.

[big cat]

I'm in the planning stage of a 54' trawler style catamaran twin eng. h/p range 175 - 200. Construction; foam sandwich, disp. aprox. 30,000# to 34,000#. I'm concidering 2 drive options; jet or prop. The jet would be North American Jet Matine they claim to have a jet that operates in the 8 to 15 kt. range that is comprable to a prop. in this speed range. The other option is a controllable pitch prop (Hundested probably). I woulds appriciate any input, Thanks.....

[Portager]

Unless North American Jet Marine has reinvented the water jet and made it much more efficient at slow speed I think you would be much better off with the controllable pitch propeller than a water jet. Here is an article that I think is unbiased and explains the trade-off speeds pretty well. http://www.camarc.com/waterjets%20vs%20propellers.pdf

The main advantage I see of a water jet for a craft this size and speed is reduced draft and appendages. The main disadvantage is propulsion efficiency.

If your Cat is a sailboat and the engines are only auxiliaries (i.e. not used to power the boat long distance) it might work OK. If it is a motorsailer or power boat, then the lower efficiency would significantly reduce range and fuel efficiency.

In the power range you are talking about I think the SABB CPP is a lower cost option than Hundested.

Cheers;
Mike Schooley

[big cat]

Thanks for the link Mike!! The info was very good.. My cat will be power only thus the higher h/p requirements. I haven't been in contact with North Americian lately, but perhaps it is time to have a serious discussion with them. I like the jet idea ( nothing hanging down to be attacked by foreign objects), but do think the CPP is more efficient... Wow the problems of a do it your self person..
Any one else have some imput??

Again thanks Mike!!!

[Willallison]

Given your displacement and power specs, I would expect a cat of that size to operate in the high teen / low 20 knot range.
As an example, I recently visited a 52' powercat to be powered by a pair of 220hp Cummins. Dry weight for the boat is expected to be about 12 tonnes (26400 lbs). Cruising speed will be about 22 knots. This boat incorporates v-drives to conventional props.
Given this, I'm not sure that the complexity and cost of variable pitch props will make them worthwhile. As far as the waterjets go, I agree with Mike, they wouldn't normally become an efficient proposition 'till your operating speed excedes 30knots or so and they tend to be very inefficient at slower speeds.

[FRANKIEFRANKIE]

You might consider www.pulsedrive.net they have a shallow draft capability like a jet but 20% more effcient and about 17% more effcient than a V Drive and since it trims that gives you the capability of changing the diameter with different loads via the trim. Also, they have a five year warranty. Check it out, they seem to meet all of you concerns for a drive unit.

[Willallison]

Don't think the operating speeds of this boat will be high enough to see any efficiency gains using surface drives. They are generally accepted to improve efficiency beyond about 35 knots - not 15 - 20

[FRANKIEFRANKIE]

Not true on trimmable surface drives. Go to the web page I mentioned and look at the testimonials at the Havasu Ferry Letter. They had a two year history with stern drives and now have a four year history with trimmable surface drives. The effciency goes from lower rpm for knots which equats to less fuel consumption, shallow draft capability, and reliability.

Take a look.

Frankie

[Willallison]

It's true that trimmable surface drives have an improved viable speed range over fixed sytems but the problem is that in order to maximise the efficiency gains available at designed service speed - say 35 knots - the propellor must be sized accordingly. This is true of any propellor, but with a surface drive, the diameter and pitch tend to be such that at lower speeds the propellor tends to overload the engine. The solution to this is to trim the drive up, lifting the prop higher out of the water. But in so doing you not only lighten the load on the engine, you also reduce the thrust available. At higher speeds this loss of thrust is more than offset by the reduction in drag, but once you dip much below the 35 knot mark the drag becomes a less significant factor.
Most manufacturers literature suggest that there is a significant efficiency gain to be had from surface drives at higher speeds - in excess of 30 knots. Some - particularly the of the trimmable variety - also claim to be able to match conventional drives at lower speeds. I'm yet to come across a manufacturer who claims significant efficiency gains regardless of speed.

I've transferred the above remarks from those I made regarding prop tunnels. I think the same points apply here.

I've had a look at the Pulse Drive site as you suggested. What is the operating speed of the Havasu ferry? More to the point how often does the vessel operate for extended periods of time at speeds other than its service speed? There is a huge difference between a commercial ferry and a long-distance cruising pleasure craft.
And given that 'big cat' is likely to operate at speeds around 20 knots and less, I can't see the justification for the considerable additional expense - but hey, prove me wrong - I love surface drives - think they're sexy as hell!

Waterjet drives have claimed to be more efficient than normal propeller, even if upgraded to skew or VP or CP. First lets reason the differences. Waterjet impeller works under hydrostatic pressure in a tunnel. The impeller inherits very high efficiency as for pumps. The bad side is the higher cost when compared to the propeller system. NZ waterjet provide a video on ship handling.

http://www.hamjet.co.nz/index%2Ecfm/...%20Videos.html

Waterjet may not be good for cargo vessels with much draft differences. Back pressure work against you. Most installation have the jet working above the waterline.

Peter