outboard powered trawlers

Hi

Can 16' diameter prop 13-16' pitch on a pair of Suzuki 150 HP 2.5:1 geared outboards sufficiently and safely power a 40 foot semi displacement (10 tonnes) trawler (eagle/defever style aft cabin boat) against current?

I plan to add knot nozzles to increase the thrust.

Currently I use a pair of 150hp ford lemans on shafts. The engines are old and I hardly get more than 7-8 knots and around 4 knots against current.

 

Does not sound like a likely proposition, is there some reason to believe the diesels have lost power they once had ? If not, you will probably lose speed, noticeably, with this outboard idea ! Not to mention, hike your fuel bill.

 

Yes the existing engines don't give me much confidence. I prefer outboard power for the short cruisers I normally do. Around 6 hours a month. Very light use.

But the tide can turn in my area and up to 5 knots. Also I get all sorts of items that get caught on the props thus engage a diver every time I need to go out which is around once a month.

Would likely sell the boat if it can't be converted to an outboard and that would be a real pity.

 

Of course it can be done, but still a big job done properly. Take all that weight out of the boat, and it will be sitting on a new waterline as well.

 

Yes. I can install the engines well and can deal with the cost and trim and water line. And I'm sure in calm conditions even a 40hp engine would propel the boat however I need to know whether the outboards would work efficiently pushing a smaller prop faster compared to the existing diesels.

Thank you again

 

No, it is not going to be as effective as a diesel inboard set-up at those kinds of speeds, but if you are fouling props continually, it appears to work on that basis alone. You will just have to go down to a low pitch prop that does not labour the engines. The depth of the props would also need careful consideration, you need sufficent immersion to avoid aeration, but not so much as to put the powerhead too close to the water, or start to get too much drag from the leg.. I think 25" leg is as long as you can get for that engine.

 

"The engines are old and I hardly get more than 7-8 knots and around 4 knots against current."

Your boat is a displacement boat and 7-8K is great performance.10ton perhaps empty and new , I would guess over 30,000lbs

Your speed over the bottom will always vary with the current.

The boat will do zero in a 7K adverse current.

I would contemplate having the engines serviced although the performance is up to par now.

Vastly more power , say 2- 400hp gasoline engines might get the boat up on the plane , say 15K where the 4k current would not be so bad.

Contemplate purchasing a much faster boat, as most rational solution.

 

Yes. I don't expect to get much more speed with this exercise (although it would be nice) nor the same fuel consumption.

Just trying to understand whether a pair of 150 hp outboards (suzukis) with 2.5:1 gearing be able to effectively power the boat?

Thank you

 

Three Bouys Houseboats out of Canada used to and may still use outboards to propel
their 40 houseboats. Many years ago we rented one for a week and they used maybe a pair of 65/90 hp motors
You could consider an email to them to see what they used for props/engines

Re the comment about the length of shaft. There is a company that makes after market leg extensions for outboards. Google Marineparts-outboard leg extensions or maybe
Bay Outboard Marine extensions.

Also some of the outboard manufacturers make High Thrust outboards with lower gearing and perhaps larger props.

But when you consider the cost of new outboards, the cost of conversion, perhaps a complete refit/rebuild might be cost effective.

In the pacific northwest, there are quite a few single engine trawlers that have a swim grid/transom attached outboard as a Get Home motor. (or trolling motor) One fellow I spoke with who had a 60 hp on a custom built bracket said he could move his 38 trawler style boat at about 7 knots quite easily.

of new higher hp diesels might also work.