Very deep V with strakes or progressive V

[jfblouin]

What is the difference (pro and con) between Very deep V (like 28 degres http://www.stingraymarine.com) with many strakes or a progressive V with smaller deadrise angle at transom and less strakes.

I'm working on 6.7 metres Rigid Buoyancy Boat whit Rigid Inflatable form and it is my first design. I have many books on design but anywhere I can't find a good technical answer. I want a boat for use in St-Lawrence Gulf (middle short wave range)

Thanks

JFBlouin

Sorry for my not so good english

[Raggi_Thor]

Other things being equal, the deep V will be less stable at rest and slow speeds, it will probably need more displacement to feel stable enough, and it will need more power. On the other side, the deep V will (maybe) give a softer ride in high speeds and high seas when you fly and hit the water with the after part of the bottom.

If you keep putting horizontal strakes on the bottom of a very deep V, what do you really wind up with ? Assumining the V is way overpowered as is usual. A boat that can develop so much speed that it could run on a long ,narrow keel area. Turn the wheel quickly and tell me what is going to happen, if you have people sitting in seats with food and drinks in their hands.

AS you are a designer , I suggest you take the time to arrange rides in ALL the competition. Nothing else will do more for a sucessfull design.

AS a side note. I have found that people who buy any type of inflatable are , trying to save money, or are the type who really enjoy a ACTIVE boat ride in all conditions, compared to rigid boats. Management has to supply what type of group you are designing for. At 20+ feet the craft has to be LIGHT in windy, wavey conditions as the bow can blow over. You are really looking at putting a lot of weight in the bow to prevent blow overs as the boat climbs over the wave in a sudden squall. Now you know what kind of weather you want on your test days.

[jfblouin]

Thanks

I want a boat for family (4 childs) soft ride at 15-30 knots in 0-5 feet (0-1,5 meters) waves and a boat able to return safetly to shore if the weather change rapidly. I want also a boat for me, to go where I want and when I want (reasonnnably) and a boat that will be enough light for trailler transportability. I'm owning a 17,5 feet (5,25 m) rigid inflatable boat but with a shallow V and the tubes are very low. This boat is very hard in rough water and alway hard for peoples in front but we love the proximity of the water and the big reserve of buoyancy.

I look to design a boat with the look of a RIB but all in GRP. I can't find a true answer on the immersed part of the boat, deep V or progressive for those two use.

[Raggi_Thor]

I do not like deep Vs with constant deadrise because they are designed for high speeds only. The best thing to do when the waves get higher is to slow down. Then you need a boat that can plane also in 15 knots. So I think you will be happier with a progressive V with a rather flat after body.

[jfblouin]

I dont want a high speed boat but I want relative soft ride. I can also accept that I need more HP for the same speed. I try to design a light boat (1500KG included 6 pass and fuel) LOA 6.7m / Beam at WL 1.8m / WL approx 6.0m. My small experience with my RIB (LOA 5.25m) is that if a boat begin to plane at 27kmh, after it plane you can slow donw up tu 23 kmh before it redrop in displacement mode. With my RIB in 1.5m waves if I go at 40kmh, I fly out the water many times. No problem for safety, the boat stay flat but very very hard for people. If I run 30kmh and when I meet a bigger wave the boat slow down below 23kmh and redrop in displacement mode. My displacement mode is 9-10 kmh and the boat is very wet. At 30kmh, I meet a wave every 1.0 to 1.5 seconde so I can't play with throttle enough fast to compensate wave. Also my old RIB have a not so sharp entry and it jump more over the wave rather cut it.

It is for what I want to design a boat very sharp to cut wave and with the capacity of running near the slower planning speed at more stable speed. My light weight disadvantage the boat but I think that low Beam permit higher V. Remenber that over the chine it have two collar for stability and safety.

Which angle suggest you?

Look one version of my design at Commments wanted on a RBB hull shape

Thanks

I hope that you will be understand my not so good english

[rxcomposite]

The two gentlemen are correct.

A deep V boat will always give you a good ride in rough water because it cuts through the waves. a shallow V (0 to 10 degree i think) will be most efficient in calm water but gives you a hard time in rough water. Ride several boats of the same size and displacement but with different deadrise to feel what is "comfortable" for you in the weather you will be riding. Deadrise effect is relative. A shallow V but heavier boat will always ride better because it does not slam too easily.

Multiple strakes or progressive "V" I think are marketing gimmick. They say it keeps you dry by deflecting the water.

In theory, as the boat goes faster, it rides up or plane.As it rides up, the uppermost chine goes up over the water and runs dry, hence the second set of strakes/chine takes over directing the water spray/separation downward and presenting a longer "wetted chine" to the water. This wetted chine makes it stable. It works only on overpowered boat or fast boats.

If you can get hold of Sorensen's Guide to Powerboats, There is an explanation on multi strakes. Take a peek.Its only on page 18.

[jfblouin]

Thanks

But what do you think between a 28 degres constant deadrise and a 24 degres at Transom and 35 degres at midship

[Raggi_Thor]

I think the variable deadrise is more what you want, but it's a very good idea to try some other boats in your home water.

I thought a CONSTANT deadrise ment 28 degrees from the transom to the area in the bow where the deadrise sharpens to the bow.

[Raggi_Thor]

Yes everything is relative, "constant" may mean from the midship to the stern. By "variable" I mean that the deadrise continue to decrease all the way aft. This makes the boat able to plane in lower speeds or maybe run efficiently in semi-displacement speeds, between 6 and 12 knots for a 16-footer. In heavy seas it would be nice to travel safely in 12 or 15 knots..

[rxcomposite]

Constant deadrise means the deadrise is the same from the point of entry to the transom. A variable deadrise or warped bottom means the angle varies from the entry point to the transom.

A shallow deadrise will give more lift than a deep one. In planing boats, this deadrise is very much dependent on the location of CG. If you have less deadrise it will give more lift. the farther it is from the CG, the more moment of arm you have. Lifting the transom will reduce the angle of attack of the hull and vice versa.

To predict the performance of the boat, you will have to use the savitsky theory and analysis that was presented in excell form in this forum. Im not sure if it dealt with variable deadrise. Variable deadrise would mean sectioning the planing areas and computing the lift V.S. moment of arm. Computation is as complicated as stepped hulls.

To make life less complicated, it is easier to trim the boat by varying the thrust angle of the outboard. if it isnt enough, they use hydraulically controlled flaps mounted at the transom.