Stability of Slow Cruising a Deep Vee Hull in Rough Water

[Astral Blue]

As some of you know, I'm in the middle of a somewhat unconventional repower. I have a Bayliner Victoria 2750 Command Bridge; and have removed the old 260 horsepower Chevy 350 and am replacing it with a 48 horsepower Perkins 4.108. My old engine was able to cruise the boat at up to 30 knots, which I seldom did. My typical cruising speed is 7-8 knots. My new engine will be able to give me a 7-8 knot cruising speed comfortably.

The boat is 27.5 feet in length with a beam of 8 feet, has a deep vee hull, squared chines, and a stern drive (concentrating the weight of the engine and the drive toward the stern). The boat's bowrise is about 4 feet above the water line.

I'm wondering whether I would be sacrificing stability in rough waters if I cruise at 7 knots. I boat in the California Delta and Bay Area, occasionally taking the boat outside the Golden Gate to either make a trek to the Farallon Islands or south to Half Moon Bay or Santa Cruz. At that speed, can the boat handle typical summer conditions on California's north coast.

A discussion I recently had with a very experienced boater who owned a model with a similar hull noted he would typically achieve most stability in rough seas when the boat is on plane, with the bow up. I have studied deep vee hulls quite well, and am aware they are considered to be the most stable planing hull with most sacrifices in the area of speed. Since speed is not an issue for me, I found it appropriate to repower this boat with a smaller engine and still maintain stability.

Just wanted to get some feedback from people here.

I attached some pics to give a better perspective of the hull's features.

Thanks in advance...

[Ram]

No idea, but it will be interesting to find out- my strong feeling is you’re not going to get that 7-8 knots cursing speed unless its flat calm and no current or seas against you- you may find you get only half that speed or less- the deep v is not the most efficient hull design for low power engines- I wish you luck

[Cheechako]

It doesnt look like a real deep V anyway.... so that might help with stability. the flatter the stern.... the less she'll roll... At least yours has some sponson on the outside giving bouyancy. If you dont have the power to plane... it will be a displacement hull right? Your speed should be about 1.41 x the sq root of the waterline length.... maybe 6.5-6.75 knots if your waterline is 23 feet. To get above that you will need to push it, consuming more fuel on a sharp increase for each little bit of speed. BTW, Bayliner made a non planing hull about 27 feet.... forget what it was called.... had a 4 cyl gas in it with large cabin like yours. Reports of instability in rough water are on the internet.... however, I think that may have been more due to a steering issue... with a 50 hp perkins and the right prop... you may be able to push it above hull speed without a huge fuel deficit... I wonder if you can find the right pitch prop for an outdrive to be used at slow speed...?
edit: I looked it up, the Bayliner "pocket trawler" I mentioned was a model 2670.

[RabidRabbit]

From what I've seen with sportfish style boats I do think they get stability from going faster than the waves/chop. When I'm motoring and rollin around pretty good the power guys come by at even just 12-20 knts and seem to power through it. Those guys burn a ton of fuel and don't have to pick as good a weather window. It seems to me that if you watch all the trawlers, sportfishers, and express cruisers it's easy to tell who wants good milage and those that have endless money for fuel. I think your plan is great and I have thought of doing such projects. I think your limited ability to swing a bigger prop wil be the only issue. The stock prop at the new fuel sipping rpm may be really slow. A vee drive and 2:1 tranny would have given you true trawler performance.

[markpierce]

My not-faster-than-hull-speed-designed motorboat has very similar hull lines except the stern is flatter and shallower (notice bottom of hull rises toward the stern).

[Ram]

Originally Posted by markpierce

My not-faster-than-hull-speed-designed motorboat has very similar hull lines except the stern is flatter and shallower (notice bottom of hull rises toward the stern).

what size engine and what speed does it do?
looks like the hull could plane given enough power

[markpierce]

Originally Posted by Ram

what size engine and what speed does it do?
looks like the hull could plane given enough power

The boat has a John Deere, 80 h.p. model 4045 normally-aspirated engine. It's capable of moving the boat a tenth or two knot over hull speed at 2400 RPM (maximum governor-controlled RPM). I normally cruise at much more econimical speeds at 1600 to 2000 RPM (perhaps 40 to 65 percent power) moving at about 6.5 and 7+ knots. The builder once offered a 118 h.p. turbocharged version, so I presume would go somewhat faster with that engine but doubt the 14-ton boat would actually get on a plane, and thus the fuel-consumption would rise astronomically.

[Astral Blue]

Thanks everyone, for the replies. This forum is a great source of information!

Originally Posted by RabidRabbit

From what I've seen with sportfish style boats I do think they get stability from going faster than the waves/chop. When I'm motoring and rollin around pretty good the power guys come by at even just 12-20 knts and seem to power through it. Those guys burn a ton of fuel and don't have to pick as good a weather window. It seems to me that if you watch all the trawlers, sportfishers, and express cruisers it's easy to tell who wants good milage and those that have endless money for fuel. I think your plan is great and I have thought of doing such projects. I think your limited ability to swing a bigger prop wil be the only issue. The stock prop at the new fuel sipping rpm may be really slow. A vee drive and 2:1 tranny would have given you true trawler performance.

I am not using the gear ratio nor prop that were configured for the GM 350 engine. The original configuration consisted of a gear ratio of 1.61:1 and a 15X17 prop. With this new engine, I will be using a gear ratio of 2.15:1 and a 16X9 prop.

To achieve the gear ratio, I replaced the lower unit on the drive with one that was intended for a 4 cyl engine.

Originally Posted by markpierce

My not-faster-than-hull-speed-designed motorboat has very similar hull lines except the stern is flatter and shallower (notice bottom of hull rises toward the stern).

The angles at the bow are very similar. This is encouraging. The main difference I see is the keel fin. I'm curious to how much of a role it plays in the boat's stability in swells. You also have a much higher bowrise, which makes a huge difference in the boat's ability to handle a breaker.

[markpierce]

Originally Posted by Astral Blue

The angles at the bow are very similar. This is encouraging. The main difference I see is the keel fin. I'm curious to how much of a role it plays in the boat's stability in swells. You also have a much higher bowrise, which makes a huge difference in the boat's ability to handle a breaker.

The keel is hollow, but the boat's thickest steel is at the keel so it lowers the center of gravity slightly. The main benefit of the keel is to protect the propeller and rudder and perhaps increase directional stability, but I don't think it increases the ability to handle a breaker.

[Astral Blue]

Originally Posted by Cheechako

It doesnt look like a real deep V anyway.... so that might help with stability. the flatter the stern.... the less she'll roll... At least yours has some sponson on the outside giving bouyancy. If you dont have the power to plane... it will be a displacement hull right? Your speed should be about 1.41 x the sq root of the waterline length.... maybe 6.5-6.75 knots if your waterline is 23 feet. To get above that you will need to push it, consuming more fuel on a sharp increase for each little bit of speed. BTW, Bayliner made a non planing hull about 27 feet.... forget what it was called.... had a 4 cyl gas in it with large cabin like yours. Reports of instability in rough water are on the internet.... however, I think that may have been more due to a steering issue... with a 50 hp perkins and the right prop... you may be able to push it above hull speed without a huge fuel deficit... I wonder if you can find the right pitch prop for an outdrive to be used at slow speed...?
edit: I looked it up, the Bayliner "pocket trawler" I mentioned was a model 2670.

My hull speed is just under 7 knots, as I have a waterline length of about 25 feet. I mostly cruise a 7 knots, and being that it is a planing hull, the amount of extra energy needed to get a knot above hull speed is insignificant compared to that of a displacement or semi-displacement hull.

The Bayliner you are referring to is the Explorer. It was a semi-displacement hull cruiser with a Volvo-Penta 4 cylinder engine (I believe it was a marinized GM 181).

[NewMoon]

We cruise BC and SE Alaska in our Bounty 257 at 6.5 knots much of the time. Our hull is considerably more deep-V than yours appears to be.

Every once in a while when 3-4 foot seas are extra steep, and depending on our course relative to the seas, we corkscrew around more than I prefer, and find it more comfortable to power up (our diesel is 260hp). But in general slow works for us. And we get 2.5 x the nmpg at 6.5 vs 18 knots.

[Tony B]

Call me old fashioned, but the only way to find out how your boat will perform is to go out in rough weather and go at 7-8 kts or whatever your intended speed will be.
Anything else is just a guess.

[Astral Blue]

Originally Posted by Tony B

Call me old fashioned, but the only way to find out how your boat will perform is to go out in rough weather and go at 7-8 kts or whatever your intended speed will be.
Anything else is just a guess.

That's what my mechanic keeps telling me! He doesn't trust well established formulas and calculations derived from them. To a certain extent, I feel there is nothing like taking the boat, putting it in water, and seeing for yourself. Just wanted to see if people here had much experience cruising at displacement speeds in rough waters with deep vee hulls. I was interested in actual experience.

[capngeo]

I have a 28' Bertram with twin Yanmar 170's. I once tried to run her from Sarasota to Key West (160nm) at 6kn turning both engines about 1000rpm. I got 1.8NMPG. On the way back at 22kn I got 1.6NMPG.... Real world numbers! Never idled her again!

[Astral Blue]

Originally Posted by capngeo

I have a 28' Bertram with twin Yanmar 170's. I once tried to run her from Sarasota to Key West (160nm) at 6kn turning both engines about 1000rpm. I got 1.8NMPG. On the way back at 22kn I got 1.6NMPG.... Real world numbers! Never idled her again!

What were the sea conditions like? And how did the boat handle? The hull on most Bertrams in this size range is very similar to mine.

Running most engines at idle is far from their peak efficiency. If you look at your engine's performance curve, you'll see running them at 1000 RPM will not provide much torque, thus causing your engine to struggle with the load when run for extended periods under such conditions. Running one engine at a higher RPM would have most likely given you better fuel economy.