Cruise 10-15kts with OB power under 30'?

Re: transom deadrise, my experience with smaller, outboard powered boats is a flatter transom allows a lower planing speed (ceteris paribus).

twistedtree said:

How does the transom deadrise play into this?

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As Broadcove mentioned, a flatter, lower deadrise run aft will typically generate more lift and plane more easily. It will also ride worse in a head sea at high speeds, particularly if you get the bow trimmed up too high. But other factors like size and placement of chine flats, lifting strakes, etc. also have an effect.

Not to hijack your thread but..

I keep my boat at a marina in Oconto, Wisconsin. Home of Cruisers Yachts. Yesterday they brought in a boat for testing, it happens two to three times a week. This one was a 50 footer powered by three 600 hp Mercury racing outboards. Rumor has it that these engines go for about $100,000 each. Supposedly will hit 56 mph.

As an aside, the engineer with the boat said that at full throttle you can go below and listen to the anti syphen valve on the gas tanks "whistle"

pete

TomCat —. I had a happy experience with the 25 - I like everything I’ve heard about the TomCats. Hope you find a nice one.

rslifkin said:

As Broadcove mentioned, a flatter, lower deadrise run aft will typically generate more lift and plane more easily. It will also ride worse in a head sea at high speeds, particularly if you get the bow trimmed up too high. But other factors like size and placement of chine flats, lifting strakes, etc. also have an effect.

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Through the 60s, low dead rise was common on most motoryachts such as Owens and Pacemaker of the day.

That started to change in the late 1950s. The Ray Hunt designed deep vee committe tender during Americas Cup trials off Newport RI changed that when a crew on a one of the sailboats - Dick Bertram - noticed how the small boat sliced through chop. He later commissioned Hunt to build a 30-footer with deep vee hull that later became the legendary Bertram 31. It did more to influence powerboat design than just about anything else in generations.

Of course, it takes a lot more horsepower to plane a deep vee.

Peter

Sweet marie said:

Whoops, I may have inadvertently hijacked this thread with my post. I thought the original post was comparing diesel vs outboard power on small trawlers. I guess that's one argument against reading in the bathroom when in a hurry.

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I remember the thread here when you were contemplating the conversion. Nice to see the result.

rslifkin said:

As Broadcove mentioned, a flatter, lower deadrise run aft will typically generate more lift and plane more easily. It will also ride worse in a head sea at high speeds, particularly if you get the bow trimmed up too high. But other factors like size and placement of chine flats, lifting strakes, etc. also have an effect.

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Well put. The east coast lobster boats that I grew up with all had very little deadrise aft and a relatively fine bow with lots of flare. They can be wet.

They don't really plane. The HP/speed curve is very smooth. There is no discernable transition. And many travel at 1.5 - 3 S/L speeds.

I've only been on an RF246 once, but I spent a few summers operating an RF18 as a kid. My club bought one in 1979 to use as a tender serving a large mooring field. With a 30 HP outboard it would go 20 mph empty, and we'd carry about a dozen passengers. It's still in service. Marvelous boat. Very efficient at all speeds, and very little wake.

Less deadrise or a flatter bottom will increase the amount of weight one can carry. And less weight will decrease the amount of power required to drive a given boat at a given speed.

An extremely low amount of deadrise or a flat bottom is seldom found on trawler craft. Taken to an extreme one would have a Hickman Sea Sled that is an inverted V hull. Combined w light weight the Sled would be a very efficient boat.

The Sea Dorys are good boats but not trawlers.

A light boat w less deadrise would be a good route to better efficiency at 10 to 15 knots. Narrow will help a lot re efficiency also and be easier to change the CG.

The running angle of the boat is very important and w a given boat changing the position (fore and aft) of the center of weight could put the efficiency speed either up or down in the range. On a light boat a bladder-like tank could jettison water and thus change the CG and all important running angle while underway. If the tank was low you could fill the tank for rough or lumpy seas or drain it for smooth running. There’s a single engined OB powered sailing craft that has that feature. But it’s a sailboat … not a trawler. It, however checks some important boxes for the thread poster tho. Boat is a McGreggor 26 if you haven't guessed. However the living space onboard is quite small tho compared to a trawler. I’d like to have one myself.

rslifkin said:

As Broadcove mentioned, a flatter, lower deadrise run aft will typically generate more lift and plane more easily. It will also ride worse in a head sea at high speeds, particularly if you get the bow trimmed up too high. But other factors like size and placement of chine flats, lifting strakes, etc. also have an effect.

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Doesn't engine tilt make this somewhat adjustable?

Sweet marie said:

Whoops, I may have inadvertently hijacked this thread with my post. I thought the original post was comparing diesel vs outboard power on small trawlers.

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Hey Sw. Marie: I don't think your post was a hijack. And it was great to finally see photos of your conversion.

I've seen other outboard conversions, but none on a boat like yours. Nice job! It seems very well-balanced.

Cheers,
Mrs. Pea Trombley

twistedtree said:

Doesn't engine tilt make this somewhat adjustable?

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Deadrise isn't adjustable with trim, as it's the angle of the V bottom. But good use of trim (engine trim with sterndrive or outboard and trim tabs on any boat) becomes important to ride quality at speed if you've got a low transom deadrise and need to make sure you're splitting the waves with the pointy bit.

Sailboats (excluding recent pizza pie boats) tend to have significant rocker fore and aft. At displacement speeds this helps keep the boundary level attached decreasing drag and allowing them speeds with engines a fraction of the horsepower of SD or planing powerboats. But they swat badly as you approach hull speed under power and if you added additional HP would swat even worse. Center consoles do as well during a hole shot but have enough HP and reserve buoyancy aft to dig out of the hole allowing the bow to come down. The Hunt hull is ingenious in that it can do this without resorting to minimal or no deadrise. A evolution Hunt has made is seen in the pilot boats built by Gladding Hearns in association with Seaton. These can operate at very high speeds but are quite efficient at the speeds the OP desires. They achieve this by being weight conscious using AL hulls and super structure with a very acute half angle. . Unfortunately they are well above the size he wants. In that size range cats make sense or smaller versions of the classic commuter hull or Sea Brights.
Sea Bright


From Wikipedia, the free encyclopedia
The Sea Bright Skiff is a box-skegged skiff developed on the northern shore of New Jersey. It was used for fishing, by being launched through the surf, sailed to the fishing grounds, and returned through the surf.

“Description
The boat is quite different from the Jersey Skiff, being more round-sided, with less freeboard, and the distinct "box skeg" that proved particularly useful when engines were adopted. The boat has a nearly straight stem, rounded sides, a narrow flat bottom that becomes the bottom of the skeg, and a heart shaped transom. -“from Wiki but commonly built at 30’ with an outboard in a well aft rather than an inboard sitting in the box keel. Ones I’ve seen are beautiful.
https://navesinkmaritime.org/page-1221444. Couldn’t find a link to outboard powered ones but they ar mentioned in this libk

To wrap up this thread...Ultimately, I purchased a Tomcat 255 from the factory (it was their demo boat, "depreciated" for tax purposes, so soon to be replaced with a new model). It was delivered on her trailer to Tacoma last October. We only had a chance to take it out twice before leaving for Palm Desert where we have spent this winter (sunny every day!!). BUT, the little time we did spend on the boat did not disappoint ) It performed beautifully...smooth and quick transition from 0 to 21-24 knots. No real sense of when we were on plane. The twin Suzuki 150s are super quiet and VERY responsive. We hit 35+ knots easily with 2 adults on board. I'm looking forward to returning home next month to ready her (new name is Easy Rider) for South Sound cruising. If we are comfortable enough for extended trips on this much smaller vessel, it will be fun to travel at 3x our usual speed and reach destinations in 1/3 the travel time! And if this summer's cruising goes well, we can think about trailering East and exploring the US and Canadian great lakes and canals over a few seasons.

Greetings,
Mr. nw. I'm sure you'll come to love your catamaran. We now have a few hours on ours and she impresses every time we go out. Yep. 0-25 and you're cruising in literally seconds. 3800 RPM gets me about 25mph IIRC.

I think the starboard throttle cable needs adjusting as she won't hold 5800 RPM (about 38MPH at that point).


She does ride better with the engines trimmed up particularly in sportier conditions. No pounding just a gentle sploosh every once and a while. We were out last Wednesday in 3' to 5' and hardly noticed. She does NOT like beam seas too much but a simple minor course correction fixes that. Enjoy.

Congratulations! That sounds like a great adventure ahead.

sounds like it will be a great boat for you... impressive speed!

RT Firefly said:

Greetings,
Mr. nw. I'm sure you'll come to love your catamaran. We now have a few hours on ours and she impresses every time we go out. Yep. 0-25 and you're cruising in literally seconds. 3800 RPM gets me about 25mph IIRC.

I think the starboard throttle cable needs adjusting as she won't hold 5800 RPM (about 38MPH at that point).


She does ride better with the engines trimmed up particularly in sportier conditions. No pounding just a gentle sploosh every once and a while. We were out last Wednesday in 3' to 5' and hardly noticed. She does NOT like beam seas too much but a simple minor course correction fixes that. Enjoy.

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RT, what brand power cat do you have...is it a Tomcat? We keep Easy Rider in Tacoma at the Narrows Marina. Where are you?

Greetings,
Mr. nw. She's a 2015 2770 World Cat Glacier Bay Edition. Twin 150HP Yamaha. We're in Florida FLL. We DID look at the C-Dory cats and for the life of me I can't remember why we didn't buy one.

nwboater said:

Today I visited the C-Dory factory in Bellingham, WA. I looked over the 25' Crusier and liked what I saw. But what really caught my attention was the Tomcat, a 25' catamaran. And lucky me, one of the owners, Mike, offered to take me out onto the bay for a ride in a demo boat. It was windy with a 2-3 chop, yet the Tomcat just bumped along at an easy 8-10 knots. There was some slapping but overall the boat was steady with zero wallowing. On the return, with the wind and waves on the stern, we picked up the speed and the ride was unlike anything I've experienced (I only know full-displacement trawler boats). We were cruising along at 20 knots with the twin 150 Yamahas running at between 2,700-3,000 rpm. They were remarkably quiet! This boat was set-up with an aft helm station, which made it easy to maneuver inside the marina and for docking, even in the wind. I am sold on these small cats

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Mt brother owns a Tomcat in Sequim. Helluva boat. I expect you would like it given what you describe as your goals here.

NWboater, please post some pics of your new toy when you have a chance, sounds like a great fit for your needs.

FWIW, the EcoTrawler might fit, 33 ft, outboard powered, cruise 12-ish knots -