Small RIB with deep V and speed potential

Hi, I'm new here on this forum. I've been reading over a lot of posts the last weeks, but there's so many that I've barely scratched the surface. So, if I'm asking stupid questions or things thats already been asked/answered I'm sorry.

To the point, I'm planning on building a little RIB (3-4m). For this project I want to design and build the hull by myself, instead of using an existing design. I'm fearly new to hull designing and I know it's a very comprehensive task. But, my main goal is to build a RIB that have very high degree of seaworthyness for it's size. That's why I don't go with existing hull design in this small size. Most of them (3-3,5m GRP hulls) have a very moderate V and I don't think many of the hulls have very much thoughs behind them (can't blame them, most of them is just a jolly or kids toys, of course, exceptions occur).

My background:
I've built a total of 5 RIBs before (5-8m). This includes molding hulls/forms, modifying normal hulls for RIB use ("cutting" them), designing/producing sponsons (welding, gluing, sewing), all rigging. I've also mounted and driven numerous RIBs and powerboats as well as wrecking and fixing them.

What I need recommendations on is basically everything regarding designing the hull and making the plug. As I've said my main effort of this hull will be seaworthiness. I'm thinking of going for a deep V (at least in the front), some strakes, and keeping the beam not to wide. I also want it to be able to handle high speeds (thinking of 30-50hk outboard engine), really would like performance in the area of 40-50 knots. That's why I've though of steps. I understand that it is quite hard to calculate the best location of these, and trial and error maybe the way to go.

So, before anyone tells me I should go for an already proven design, I know I'm probably in over my head, but that's whats make it exciting. This would just be a fun boat and I think because of the size and all it would not be totally economical unwise to do this as a fun-test project.

What I would like to know is a good program to design and run hydrostatic/speed/seahandling calculations in. I've looked at Maxsurf (they have a educational version) and Freeship. Next, I would like advice on best materials to use to make the plug. Thinking in the lines of different kind of wood (plywood/balsa) and/or some sort of foam. The final hull/plug/form will be in GRP. Also, it could be interesting to make a scale model, but I understand that scaling doesn't give a 100% accurate result. What would be the best scale for a model? Also, seatrials with fullsize would probably be wise, with temporary steps/strakes.

Also, I understand there's a few standard books I should get before I start to design, and I will order a few books this week.

So, any input would be highly valued.

 

This sounds like a very interesting project.
My suggestions would be, don't fool around with models for this small boat. Just build the boat.
I really don't think you should go for a deep V or steps, at least not a very deep V.
But I really like your idea for building this sort of boat.

 

Why wouldn't you go for a deep V? I think I would at least have sharp V in bow section and probably have a variable deadrise V-shap to the transom sharp to medium or something.

I have seen a few successfull boats having something like a V-ski or something. Don't know quite how to explain it, but I've tried to make a sketch (yes, a very bad one). Often this center-V ends 20-40 cm before the transom, making less wet surface aft once on plane.



Hull shape exaggerated.

 

Did you think more deeper V will bring higher speed?I think it depends on the boat width.

You are right.Reading more books will help you to make a good decision.

 

I don't think a deeper V would mean higher speed. I think a deeper V generally gives a softer ride, but needs more power to achieve same speed. Since I'm thinking of 40-50 hp for this boat it would be "overpowered" compared to similar RIBs (normally they have 15-25 hp). What I'm mostly interested in is seaworthyness, but after overpowering several other RIB's I know that not all designs can handle extra power (deep V-hulls) without starting to wobble or become unsafe.

I'm also finding it a bit hard to decide which book(s) I should order regarding hull design. Any tips would be valued.

 

Making small RIB with 40- 50hp,security is a big problem,the boat should be strong enough,have you think about building your boat in the way of vaccum infusion?

 

I've considered it, but I have no former experience with it so I must investigate more closely. As far as getting the boat strong enough, I think it would be no problem, since I have a bit experience with overpowering and wrecking boats of similar size. Of course I've fixed them and then they could handle all the punishment I threw at them. I've found out that normal boat strenght just doesn't work in extreme conditions or very hard driving.

Any tip of strengthening the boat would be greatly appreciated. I think with normal molding with fiberglass and rowing strengthened with divinylcell/plywood "sceleton" it would be strong enough. It would maybe weight a bit more than a normally for this size, but it should be a fun boat for rough conditions.

 

My reasoning for not recommending a deep V has to do with my experience with the type, which is rather limited. Their virtue is that they will normally perform really well in what I would call large choppy water relative to their size. But the downside is they maneuver very poorly at low speed and once the waves get big enough to prevent safe high speed operation they wallow like a hog while using a lot of fuel. If pushed under these conditions (not planing) they tend to practically stand on their transoms so you can't see where you are going. They also draw more water than a more conventional boat. Also the wake is not suitable for skiing. These are characteristics I just don't think I would like to have in a 3 to 3.5 meter boat.

 

Points duly noted Gilbert.

 

I used to work for a infatable company and we found a nice glass hull almost the same as what you are looking for and took a thinnish shell of it . From there it was set up over a tube and the build process done from there .By using a existing hull you have a pretty good idea as to its performance before you get to the water , Its only from 100mm above the chine down you need ! Deep vee needs lots of power to push it !! you also need to get the tubes well clear of the water when it going as that slows it a lot , build it strong but flexable as thats where most companies fall down and the hulls crack or are built far to heavy and weigh a ton almost .

I maybe a junior member but i started in glassing in 1972 and built some really fast IRB surf boats all from glass and never had one break ever !!

 

designing a V hull should not be a problem. Flatining it back to the transom
is a math problem I will not attempt to advise. You want this boat to plane up and out of the water. Determining the D.W.L. at rest is another math problem but easily solved. You want the sides up some 3" to attach the tubes to with a solid continous washer and bolts. The tubes must be out of the water at cruise with, perhaps only the aft part touching water. I suggest you look at wood plans for run a bouts and examine the hull to get youself educated about the planning design and D.W.L that you can use as a guide for a fiberglass mold. You might use a built wood hull with short sides as a mold...Tube size is determined by weight of hull plus engines, fuel, batteries and passingers. Good Luck.

 

Best to look at boat reports and find a nice looking boat that has a good performance write up and take all you measurments from that then you are starting 3 steps ahead as its a known quantity .
Changing the shape of a hull with the thoughts of trying to improve it is a waste of time unless you really know what you are doing and 100% know what works and what wont . Playing with small hulls can be and is dangerous if you get it wrong .

 

Here at Explorer Marine we build fast custom RIBs - see www.explorermarine.co.uk - and we run three different hull forms for different purposes. We build a SPORTS model with a 15 degree deadrise at the transom, for fast work, a DIVE model with a 22 degree deadrise for heavier loads and more offshore work, and an XTREME model with a 28 degree deadrise for very heavy seas and a gentle ride, or for high level wakeboarding where a big clean wash is required.. Deep vee hulls really absorb engine power so if you want a small boat with some seakeeping properties, have a deep forefoot leading to a medium or shallow vee astern.
On a boat as small as 3,5 metres the issue will be dynamic stability under different LCofG conditions. Generally boats of that small size are very unpredictable at speed because the mass of the crew and load is a very high percentage of the total and thus the balance can be easily upset. Have a study of the hull dynamics at high speeds - our boats are stable to 50 knots, and can exceed 60 knots; but many - even famous brands - will generate a horrifying instability, usually porpoising and / or chinewalking, at much lower speeds.
Unless there is some practical reason to make a boat this small I would strongly recomend that you move to a bigger boat. Our 5 metres cost very little more than the smaller sizes - the tube is the largest cost of a small RIB and tube costs are not linear with boat size. Equally a larger boat with a suitable hull will go faster than a very small overloaded hull, even with the same engine.
Good Boating