40 Horse Outboard on Catalina 30?

[thomm225]

Originally Posted by Hud3

Something to consider is that an outboard will come out of the water in certain windy, choppy conditions, limiting it's ability to push the boat. A friend of mine ended up on a New Jersey beach because of that phenomenom.

I motor sail in those conditions. Driving the boat somewhat off the wind and let the sails stabilize the boat.

It works but was quite hard once coming out of a tight channel. I ended up just sailing out because the wind finally came up enough..........

[thomm225]

Originally Posted by jongleur

Thanks.

Maybe I should buy the 40, sell it and get a
newer 15?

Actually, you could skip the buy and sell the 40 part.....................or did I miss something from earlier

[d design]

Also consider that you dont want to have to run your outboard at full throttle but half throttle for less noise, more efficiency, and longer service life.

[valhalla360]

I know it's not a direct correlary but there is extensive discussion and testing on Gemini catamarans.

Assuming for the moment that there isn't an outright problem with the installation (structure can't support the motor, it gets dunked or the prop is always out of the water, etc....)

They were originally outfitted with 40hp outboards. Most have switched to 25hp when the time came. A few have dropped to 9.9/15hp. (We switched from 40 to 25).

The biggest issue is the 25" shaft is hard to come by larger than 15hp but smaller than 40hp. (You can get after market extensions at a cost for a 25hp)

The small motors have to be run flat out to get hull speed in calm conditions. If you are fighting a head wind or waves, you can have trouble making headway. Around the docks, you have less power available.

At 25hp with a workboat prop, we actually found we had more thrust available when docking. Up to around 25kts headwind, we can maintain hull speed (bays and protected water where waves can't build up). After that we do start to lose speed if the wind gets higher.

You didn't mention if the 40hp was 2 or 4 stroke. For a sailboat 4 stroke is much nicer.

[Don1500]

Thanks for starting this thread. I was having a hard time wondering why my transom was so low in the water when cruising since I had the transmission changed. I realize now that I can now reach my max hull speed of 7.5knts. When that happens the keel sits in the trough between two waves and the bow and transom are low in the water. First time I saw the transom so low I thought I was taking on water. At the time I also noticed that the following wave remained in the same place relative to the boat. This was in flat water.


http://www.cncphotoalbum.com/technic.../hullspeed.htm

[thomm225]

[QUOTE=valhalla360;1394790]
The small motors have to be run flat out to get hull speed in calm conditions. QUOTE]

With the 5hp 4 Stroke Mercury (25" extra long shaft) Outboard on my 6600 lb Bristol 27, the speed stays about the same from around 1/2 -3/4 throttle on up so I usually run it just past 1/2 throttle.

Depending on the tide and wind it will push the boat 4-6 knots. If there is a tiny bit of wind, I'll use either the main or the jib also depending. I can cross the 17 miles of the lower Chesapeake Bay with around a gallon of gas.

It takes a few hours but using the autopilot it's a nice scenic ride. I can usually spot the amphib ships and the hovercraft port (my reference points for Little Creek) from about 10 miles out.

[cruisincanucks]

You have a displacement hull....40hp has huge weight and is grossly overpowered for the sailboat. You cannot go faster then hull speed and the support on the stern was not built for that type of engine....don't do it...just my 2 cents.

[Cheechako]

15-20 Hp would be great for that boat. A new hi thrust 8 or 10 would be great also. Wh not 40HP?
* Cost/size of the large engine bracket ( a regular 10 hp bracket wont work)
* Is the transom beefy enough to hold that bracket and motor or will you stress crack the fiberglass?
* The engine will suck fuel fast... how big a fuel tank do you want in the cockpit?
* Need to wire in the electric start and battery. However you may want this for the smaller engine too.
* Al that weight hung out there on the stern will make the boat hobby horse a lot in chop.

[jongleur]

More great info.

Thanks.

[vancouver25]

I use a Tohatsu 6hp Sail Pro on a Pearson Triton in an outboard motor well. So slightly less displacement (~9000 lb. in cruising trim). But also full keel so maybe same wetted surface as the Catalina 30?

Anyway, 5 knots cruising whether at 3/4 or full throttle in calm conditions. Up the channel against 25 knots wind with the tide about 1/2 knot against us we barely make 3 knots WOT. But, wouldn't you rather sail if there's wind?

Only problem would be if you expect current and chop...6 hp is not enough to punch through anything and there are cavitation problems if the chop is short and steep.

Fuel economy of a modern 4 stroke is probably close to a diesel. I use about 1/3 gallon an hour.

I don't think I'd want to deal with the weight of even a 9.9 hp when it comes time to remove/move it and anything over that seems almost dangerous.

[idylours]

with the cock pit layout of the cat 30 an outboard would be a pain. just change the diesel in the end the cost will be negligible, and the resale much more.

[valhalla360]

[QUOTE=thomm225;1394870]

Originally Posted by valhalla360

The small motors have to be run flat out to get hull speed in calm conditions. QUOTE]

With the 5hp 4 Stroke Mercury (25" extra long shaft) Outboard on my 6600 lb Bristol 27, the speed stays about the same from around 1/2 -3/4 throttle on up so I usually run it just past 1/2 throttle.

Depending on the tide and wind it will push the boat 4-6 knots. If there is a tiny bit of wind, I'll use either the main or the jib also depending. I can cross the 17 miles of the lower Chesapeake Bay with around a gallon of gas.

It takes a few hours but using the autopilot it's a nice scenic ride. I can usually spot the amphib ships and the hovercraft port (my reference points for Little Creek) from about 10 miles out.

Assuming a 25' waterline, hull speed should be around 6.5-6.7, so you are only making 4-6kts, you aren't at hull speed.

Also, it's not a linear relationship. On a boat that is 40% lighter and 20% shorter, you need less power to reach hull speed, so you getting by with a smaller engine doesn't tell us a lot.

You can get away with a lot of things in terms of power. An engine that will comfortably take you up to hull speed in calm conditions is just a data point but it is a farily commonly used one to balance enough power without going overboard.

[SkiprJohn]

[QUOTE=valhalla360;1487760]

Originally Posted by thomm225

Assuming a 25' waterline, hull speed should be around 6.5-6.7, so you are only making 4-6kts, you aren't at hull speed.

Also, it's not a linear relationship. On a boat that is 40% lighter and 20% shorter, you need less power to reach hull speed, so you getting by with a smaller engine doesn't tell us a lot.

You can get away with a lot of things in terms of power. An engine that will comfortably take you up to hull speed in calm conditions is just a data point but it is a farily commonly used one to balance enough power without going overboard.

Bristol 27 LWL is about 20 feet. Hull speed is 6. He's doing great from 4-6.

[SkiprJohn]

I once met a man who installed an Olds V-8 diesel in his 40 foot sailing vessel. He said he wanted to go fast. I just left the pier shaking my head.

[valhalla360]

[QUOTE=SkiprJohn;1488057]

Originally Posted by valhalla360

Bristol 27 LWL is about 20 feet. Hull speed is 6. He's doing great from 4-6.

Fair enough but as I said, it's no a linear relationship between HP and length, so cutting 1/3 off the waterline length doesn't mean a direct 1/3 off the HP requirement.

Also, what is the LWL when at hull speed? I'm not familar with the particular quirks of this design but a lot of mono's used to have overhangs that at dock resulted in a short LWL but once you got them moving, the effective LWL got longer. This is something that is not as common or at least far less pronounced in cats.