What are the engine-size limitations on a Cat?

[boatingnewbie]

I noticed a new 40 footer from FP has 20HP engines that can be upgraded to 30HP Yanmar from the factory.

I'm very new to Cats and was wondering what sort of engine limitations one was faced with. For example, could that 40 footer (that can be upgraded to dual 30HP engines at the factory) handle 50HP engines instead? What about 75HP? I realize fuel economy would suffer, etc. for extended motor cruising, and/or would need a larger fuel tank, but just wondering if anyone has put much larger engines on their Cat?

As for WHY someone might want larger engines... maybe to get away from oncoming bad weather faster or to cruise at a faster rate of speed for a certain distance if trying to reach a port before dark, etc.

[cal40john]

http://www.cruisersforum.com/forums/...or-190132.html

[StuM]

Look at the power/fuel curve on a typical cat diesel and the small increase in speed with lots more HP (and fuel consumption) once you get beyond 6-7 knots. It soon becomes a bit pointless.



With my twin 38HP Yanmars, my figures are.



Knots.........HP ............lph

7.0.............22............5.8
7.5.............30.............8.2
8.0.............50...........13.4
8.5.............75...........20.5

[valhalla360]

Yes, you can (with proper installation which may include reinforcing the hulls and redistributing weight to keep the boat in trim).

Many of the early trawler cats were basically just a sailboat hull with the mast removed and bigger engines installed.

The problem that comes in is the fuel usage goes up so fast as speed increases that for the most part outrunning storms offshore doesn't work as you will run out of fuel too quickly.

[Cadence]

Most of them are like any displacement hull. Once hull speed is achieved you are pissing up wind, so to speak. A sailing multi can often make far better time often under sail but consider where the energy is applied.

[Mike Banks]

My tri was faster under sail than I could have motored at full speed. The sails generated about seventy horsepower more or less, --the engine about 25.

The sails and masts weigh less than the motor, gearbox and controls. Which is why many cats use outboard engines. About the same in weight as a small diesel engine--when not in use one can lift them out of the water. Unfortunately, small high-speed diesel engined outboards are no longer made--but some heavy duty models still exist.

One can slide over the water or cut through it. If you are planing, which most sailing vessels do not (although some racing craft do) you will be limited to the hull speed of the vessel for the most part.

Moving through the water involves all sorts of hull designs from those laminar flow profiles which gain as much impetus from the water closing behind the hull as is required to open it--such as occurs in nature in a shark's body--or it can just be a slow barge designed to carry the heaviest loads with some free board--and move only as quickly as the water can get out of its way, and overcome the drag of the turbulence behind it.

So--there is no easy answer. My vessel was happy at seven knots--more than that used excessive amounts of fuel--but I could get eight to ten knots depending on wind direction. Best I ever got under sail and overloaded was fourteen knots--and that was a thrill.

[Cadence]

Originally Posted by Mike Banks

My tri was faster under sail than I could have motored at full speed. The sails generated about seventy horsepower more or less, --the engine about 25.

The sails and masts weigh less than the motor, gearbox and controls. Which is why many cats use outboard engines. About the same in weight as a small diesel engine--when not in use one can lift them out of the water. Unfortunately, small high-speed diesel engined outboards are no longer made--but some heavy duty models still exist.

One can slide over the water or cut through it. If you are planing, which most sailing vessels do not (although some racing craft do) you will be limited to the hull speed of the vessel for the most part.

Moving through the water involves all sorts of hull designs from those laminar flow profiles which gain as much impetus from the water closing behind the hull as is required to open it--such as occurs in nature in a shark's body--or it can just be a slow barge designed to carry the heaviest loads with some free board--and move only as quickly as the water can get out of its way, and overcome the drag of the turbulence behind it.

So--there is no easy answer. My vessel was happy at seven knots--more than that used excessive amounts of fuel--but I could get eight to ten knots depending on wind direction. Best I ever got under sail and overloaded was fourteen knots--and that was a thrill.

One of my tris (SR31) would get a comfortable 7.5 kns at .75gal with a 15HP Honda 4stroke 28" leg. She got 17.5 kns once under sail. Something I would not do again. The windward ama was farther out of the water than what made me comfortable. I think the 25 Piver would sail faster but was pre GPS.

I'm not sure a tri and a cat are comparable other than not being a mono.

[44'cruisingcat]

If the hulls are designed for a sailing cat, they probably won't have enough buoyancy aft to carry much bigger engines and also handle the squat caused by the thrust of them.

Power cat's hulls are designed differently.

[atoll]

and then there was the CF member that put 2 X 180 hp outboards on the transoms of his 35 ft cat.......i can't recall his user name but the video he shared showed his cat heading for home at about 35 knots!
perhaps someone has a link

[Mike Banks]

The tri and cat are comparable in that neither of them are dragging two or more tonnes of dead weight through the water, and drawing an extra half-metre or so of water depth, just to make the masts stand up. Their requirements for engines are therefore similar, but it is easier to mount a 30 or 40 hp diesel engine in a tri--with all of its weight and more limited accessibility accessibility.

So- Cat and Tri displace about the same amount of water--and require similar amounts of engine power. Neither of them are designed with planing hulls. To make them point better, some have centreboards as do some cats.

My tri had the Cross mods of a long keel and two stub keels on the amas, so she sat nicely on a beach--and was powered by a 27 horse power (nominal) Yanmar. Two outboards, as many cats employ, would have been better when sailing or manoeuvring. Tris in a crosswind are not easily handled in a crowded marina unless deck crew are available, or the vessel uses outboard engines which have reverse capability.

The displacement is about the same. The wetted surface on a tri is probably a l;ot more however--but then--they are I think more comfortable--but back to the question,

Are outboards better?

For a cat or tri of 40 feet or smaller, I think YES.

Engine sizes--20 to 30 hp four-strokes of any Japanese make down to about 9.5 Yamahas. I like the Japanese ones because of the better serviceability and availability of spares. Bigger is heavier, and two strokes use too much fuel unless they are the larger turbocharged dry sump fuel injected variants that do not mix oil with fuel.

[44'cruisingcat]

Originally Posted by Cadence

Most of them are like any displacement hull. Once hull speed is achieved you are pissing up wind, so to speak. A sailing multi can often make far better time often under sail but consider where the energy is applied.

Cat's usually aren't really limited to hull speed. Boats with LWL: BWL over around 8:1 don't generate the big bow wave that produces that limit. They can exceed theoretical hull speed without planing, and without needing the huge quantities of additional power.

[Shaneesprit]

Put weight on the back of your CAT and spoil its sailing,
CATs have lots of room but like to travel light.

[Cadence]

Originally Posted by 44'cruisingcat

Cat's usually aren't really limited to hull speed. Boats with LWL: BWL over around 8:1 don't generate the big bow wave that produces that limit. They can exceed theoretical hull speed without planing, and without needing the huge quantities of additional power.

I was not speaking theoretical hull speed but what a specific cat will do, under power. past that you are just adding weight and digging a hole under the stern.

You are correct the mono theory of displacement speed does not apply to cats or some tris.