Trimaran - Especially Searunner - Owners

[owly]

Plusses and Minuses:

On the Minus Side:

1: Barnacle and other growth on the prop &c is not a problem with an outboard that is lifted.

2: Steerability of an outboard is an asset.

3: Setting up a shaft that runs true, installing a shaft log and stuffing box is a significant project.

4: Prop and shaft more vulnerable than an outboard

On the Plus Side:

1: A cheap rugged, and reliable 4 cycle industrial engine of 10.2 HP ($700 retail) with readily available and inexpensive parts. It's replaceable virtually anywhere in the world.

2: The prop would always be submerged where it is effective.

3: Engine is not directly subjected to seawater internally or externally, and chances of the engine being swamped by a wave in the cockpit are extremely small. Corrosion from salt air, and spray will still exist, but with a cover over the engine most of the time, this will be minimized.

4: 4 cycle engine efficiency, full pressure oil system, electric start option

5: Throttle and shift virtually in the cockpit, meaning no long cables, etc.

6: Drying effect from heated air exhausting beneath the cockpit sole

There are probably other plusses and minuses. In my mind the two largest minuses are the vulnerability of the prop and shaft in a shallow draft boat. The prop, a folding prop presumably, will be somewhat protected by the mini keel that most SR31's have, and the skeg. The need to access the prop and clean the build up, as well as having and maintianing suitable sacrificial anodes. I dislike anything penetrating the hull, and see zero good reason to have ANY through hulls. The shaft log of course violates this. An outboard can eliminate all need to bring any water into the boat from below the waterline..... or discharge anything below the waterline. An ordinary inboard requires water inlet, and water and exhaust outlet, with at least the inlet being below the water line and vulnerable to leakage / breakage.
The ability of the Searunner 31 to be driven into shallow water and beached at high tide......... to "take the ground" without damage, can be an important and even a life saving feature. The damage boats sustained in the recent hurricanes and how that damage was sustained, really drives home the value of shallow draft. Much of the damage, as usual, was from impact of break away boats. Drive the boat far into the shallows, and tie off to mangroves, and you are beyond the reach of most of those "loose cannons".

H.W.

[Cavalier MK2]

My cheap and easy air cooled longtail suggestion would be to attach a motor box/fairing for a horizontal aircooled engine under the short wing section on the 31. Nothing in the main hull but an outside addition so you could place it anywhere for and aft for less cavitation and if it didn't meet expectations or you want to change it is easy to remove/cut off.

Here is a traditional crude back of envelope sketch. You're apt to get into decent outboard prices on what you want to try between machining, exhaust systems etc..... Maybe you could find a 31 that already has the inboard and shafting installed to convert. My take would be to make sure you are tinkering for fun because it is going to cost some money unless you are set up for the mechanical fabrications.

[Jimske]

Originally Posted by owly

Below....... If I can get them to come through are two photos. One of an SR31 with a sort of "sled", and the other showing the rear strut assembly on another open wing SR31. The question in the second photo is where do you put the "sled"?
What it all boils down to is that there are no easy answers...

H.W.

The pics are what my SR31 looks like. I wouldn't mind having a 20HP with the same weight. It would give a bit more umph in flat water where the current is an issue (New England). At 3500 RPM in flat water, I can do just under 5 knots and estimated last week .4 gallons an hour for 6 hours. In such conditions I'd like to do 6 or 7 knots but even at 5000 rpm I can't get 6 knots and fuel efficiency goes way down so I try to keep it less than 4200 rpm.

My SR31 goes pretty close to the wind and is fine if under sail but no fun powering into any kind of sea. So except in rare circumstances I don't.

Electric? Perfect light weight boat and simple installation under the stern double. How about a couple wind generators, some solar and some lithium power? Keep the lateral lift. Use both or just one. The outboard also charges the batteries.

J

[Cavalier MK2]

Jimske, I use a Yamaha 9.9 high thrust workboat outboard. They have a 3-1 reduction and turn at low revs like 2200- 2500rpm. Cruising at 5 1/2 knots I'm burning a 1/2 gallon an hour. Wound up we hit 7 knots. They've built these things forever so they are easy to find. I did the math and found there was very little prop slip. There is plenty of power to punch into bigger wind and waves. The Nicol is quite a bit bigger than a SR31 and I don't need more motor.

[Boatguy30]

Re: stern castle outboard

I believe that was a SR40 built in FL to supposedly pirated plans. Guy supposedly built 5-6 searunners which if true, should perhaps qualify for free plans anyway.

That boat has a 60 hp. Capttimbo or something is current owners handle.

[Jimske]

Originally Posted by Cavalier MK2

Jimske, I use a Yamaha 9.9 high thrust workboat outboard. They have a 3-1 reduction and turn at low revs like 2200- 2500rpm. Cruising at 5 1/2 knots I'm burning a 1/2 gallon an hour. Wound up we hit 7 knots. They've built these things forever so they are easy to find. I did the math and found there was very little prop slip. There is plenty of power to punch into bigger wind and waves. The Nicol is quite a bit bigger than a SR31 and I don't need more motor.

I have a Suzuki DF 9.9 btx High thrust 4 blade 10 x 5 pitch. So you're saying that if I change props I can get better performance?

[Cavalier MK2]

Originally Posted by Jimske

I have a Suzuki DF 9.9 btx High thrust 4 blade 10 x 5 pitch. So you're saying that if I change props I can get better performance?

Most likely. It will be next week before I pick up my copy of Skene's Elements of Yacht Design which has a decent prop section. If you don't have one I'd be happy to run the charts and numbers then. What I'd need are the designed rpm range that motor is supposed to work in. It sounds similar to the Honda high thrust unit that uses a 2/1 reduction gear with a faster turning prop. In my experience this usually means a bit more slip than a big slower prop. My old Yamaha is a 3 blade but there is virtually no slip in a calm. The other thing you should check are what prop sizes are actually available from Suzuki. There are only a couple for the Yamaha.

[Jimske]

I think it says 2.08:1. I'm going to check with dealer. Seems several props available.

[owly]

The searunners list mast height as so many feet from "trunk"............ Does "trunk" mean the step? For example, the SR31 is 30' "from trunk", and it is stepped on the cabin top, the 34 is listed as 43' "from trunk" and it is stepped on top of the centerboard trunk a foot below the cockpit sole, as are all the larger searunners. Does "trunk" refer to the centerboard trunk as a standard point of measurement, or does it refer to the mast step itself? In other words, is the compression post in the SR31 included in that 30'?? H.W.

[Cavalier MK2]

Originally Posted by owly

The searunners list mast height as so many feet from "trunk"............ Does "trunk" mean the step? For example, the SR31 is 30' "from trunk", and it is stepped on the cabin top, the 34 is listed as 43' "from trunk" and it is stepped on top of the centerboard trunk a foot below the cockpit sole, as are all the larger searunners. Does "trunk" refer to the centerboard trunk as a standard point of measurement, or does it refer to the mast step itself? In other words, is the compression post in the SR31 included in that 30'?? H.W.

Your figures are wrong for the SR31. The actual mast length is 35'. Measured on the mast, or from the cabin top if you prefer.The plans of the boats all give spar lengths on the rigging plans. The trunk refers to the centerboard case that most step on.

[md7a]

I think the mast measure for the 31 is from the step, on the cabintop, not including the post. I don't have the measurement handy from my boat, but I think the main halliard hoist was 32' not including about 10" at the top for the tackle and about 24" at the bottom, and my mast is a little short. It is definitely longer than the boat by several feet.

[Cavalier MK2]

I had a 31 and have the plans. It is 35' on the mast.

[owly]

This is right from the Searunner website............ Left me a bit confused as I believed the mast height on the SR31 to be 35 above the cabin top also. If it were above the centerboard trunk, it would be a short mast indeed. If the other mast dimensions are from the centerboard trunk, that makes the actual height of the SR31 mast from the centerboard trunk about 39.5 feet, compared to 43.5 feet for the SR34 and 45' for the SR37, far less difference than I had imagined.

[Cavalier MK2]

One of those anomalies you get when you try to stream line things. There is a slight difference in mast length between the wood mast and a aluminum one to allow for the difference in masthead gear/truck but the overall height and headstay lengths come out about the same.

[rossad]

Designers often have two mast heights. I have seen a SR37 with a mast 3 ft higher than mine on my SR37. It was for a racing and certainly was a quicker boat than mine. That SR37 was actually called "SEARUNNER". Built in NZ and is now in Vau vau Tonga. But because i am not wanting to race Im still happy enough with the boat speed. As soon as the wind is over 15 knots it doesnt really matter anyway.
Cabin tops vary so I think where the centre board case is and where the mast steps onto that plate that would indicate measurement point.
Higher mast makes more sense in lower wind speed.