Choosing building materials for a 22' powerboat

Hi,

I've been dreaming (by the way, please excuse the long read and bear with me) for a while of designing and building myself a 22' GRP powerboat, and have come far in the design process (with good help from a couple of Gerr books and studying other, similar crafts), and now I'm trying to look into which building materials to use. Luckily my father have built several smaller (GRP) boats, the largest being a 15,6' fishing boat (medium speed, 31-32 knots), but as this this is in a completely different league I'm searching for some guidance here (I've been sneaking around here for quite a while gathering information, but I haven't found much being enough specific).

As said, the result will be a 22' GRP powerboat, deep vee (24°) and capable of handling up to 250 outboard HP's (which should push the boat up to about 55-60 knots). Attaching an image of the current design if that might be of help.

My specific questions are:

Is the combination fibreglass mat/woven roving suitable for this kind of boats? I understand it comes mostly down to hull thickness vs. strength of materials (and some other factors), but other than that I don't know that much really. Our 16,5-footer were built exclusively of fibreglass mats (regular ones with chopped fibres everywhere), ended up with a weight of about the same as of mass-produced comparable boats and has taken a lot of hard beating out on sea (try 2 metre waves in a 16,5' fishing boat for the first time and the term "beating" will get a new dimension). Does this mean that using only mats on the 22' will be good enough? I'm pretty sure it won't if I'm interested in keeping the weight moderate, but please enlighten me.

Also, my father have - in all his years of building different kind of smaller GRP boats - always been using ortho-polyester only, and he's never seen much evidence of blistering, perhaps because of the cold seawater and climate up north, but also perhaps because of proper finishing. But what about the 22'? Obviously it will be victim of a h*** of a lot more beating, so ortho-polyester is perhaps no good considering that it has quite a bit lower flexibility? I don't consider osmosis to be a big threat, so that's not an important decision factor. What about iso-polyester then? It's a lot more expensive I admit, but it's also better in every way than ortho-polyester. Do you recommend using this resin? Or to put it in another way; Do you strongly recomment NOT using ortho-polyester?

Anything more expensive than this - both concerning fibreglass and resin - higher-quality solutions are more or less out of the equation (I'd rather sacrifice some weight than increase the cost significantly), but any arguments/help are highly appreciated. Nothing helps more than the viewpoint of professionals/serious amateurs.

 

If you read Gerr it says that the book's formulas work only up to 40 knots. Go back to the drawing board. You are asking about materials without context. That is, any resin and type of fiberglass can work for the application. Each will have a different lamination schedule.

 

Thanks for the reply. As said, I've come far into the design, but failed to mention that I in no way see it as complete. If the approximations from Gerr's "boat strength" (yes, I read the speed limit sidenote for the thumb-rules) are close to the actual structural requirements required I think it's a sound idea to ask around here. (i.e what kind of materials/combinations would typically be used in a craft like this to make a sturdy boat in the "cheaper" segment and with flexibility regarding weight?)

I'm not hoping to receive a complete build-plan for my boat in here, but rather pointers for the next steps.

Regarding the structural requirements, I'll calculate the final numbers after deciding which materials to use (with NBS (Nordic Boat Standard) and possibly a few relevant ISO-standards. Not too great a challenge for a construction engineer I'd guess). Of course, there are difficulties ahead that needs to be solved, but I like a good challenge. Even more so if I learn anything from it.

 

No one in here willing to share their knowledge? What about helpful books on the subject of considering different materials and grades?

 

Ronny, the general assumption is to dunk a bunch of materials into a hull and there you go. There is no quick cheap fix. You will have to go to a naval engineer and have him work out how many layers of what and how they must be layered up in the hull as was suggested by Gonzo. There is more involved than meets the eye.

A hull like yours have to be made very strong. If you can imagine looking at the bottom, at speed, can you imagine the extreme colission impact of the water on the hull surface ? The faster you go the harder the water becomes... try jumping into the water at 100km/hr, you can just as well jump out the car into the road at the same speed.

If you make the hull wrong you make an accident waiting to happen. You are creating a formaula one car on water but no engineering...


BTW, it seems your hull shape remains the same front to back. If the profile doesn't change towards the transom the boat won't plane upright, it's going to flop to either side and out of control. The cost of a naval guy is going to be like nothing compared to the cost of the boat anyway, and be money well spent.

 

I thought the whole thing looked too easy after reading a bit in Gerr's books, but of course you are correct. Although I'm a construction engineer I've got no experience within naval design, and the result could easily be like doing russian roulette with a full barrel.

Gerr's "Boat Strength" specify 40 knots as the upper limit, and the difference from 40 to 60 knots in terms of wave impacts etc is actually 2.25 times. So a "redesign" of a hull with an absolute maximum safe operating speed of 40 knots needs to be 2.25 times stronger with an absolute safe operating speed of 60 knots. And that's just for the wave impacts.

I suppose I got a little carried away, but as you say it might be very wise to get in touch with a professional. The only problem is that I've got no idea where to find such a guy. And how much can I expect to pay?

Regards
Ronny

 

It needs to be more than 2.25 times stronge. To make the bottom stronger it will be heavier, which will make it hit harder. Because it is heavier you need more horsepower, a bigger engine, heavier too. It is the typical design spiral.

 

That's true, Gonzo. I wasn't taking the added weight into account, merely imagined a speed increase from 40 to 60 knots.

Regards
Ronny

 

Ronny, we have some lunies fishing here with boats that can do that 60kn on the water. One would swear we have flying fish here and they have to catch them (pun).

Some of the boats can do it, it's not very pleasant at that speed so if you think of taking the missus or any one for that matter along think again.

Different hulls can work at different max speeds. My boats hull can work to about 40kn, it is where it begins to feel less stable, some of the other guys have much faster hull's. Doesn't make them better fishermen either.

So you'll have to design the hull to cope with the speed. One thing I like is you're looking at a longer boat, some wants the 4m800 long hull to run there.
Another thing is you'll have to hang a really powerfull motor on it's behind, probably fiddle a bit with it too depending which motor you choose. It's not going to be cheap either way, and remember, there is always going to be another yappi with more money with a faster boat and may I add less brains.

There are very few things one can enjoy alone, but there are lots one can enjoy with other people, friends, family, kids or even the old witch if you can get her off the broom.

Persnally I think you're a bit nuts for wanting to run around at that speed, which is outright dangerous. If you're looking for adventure and a thrill, then get a decent sail boat. I was into power boats all my life, only off late discovered how nice it is to sail. Almost too figgin late.

Myworthless za 2c.

 

Thanks for your "worthless 2 cents", Fanie. I think you made some very good points there.

What's really the point of having a boat able to reach 60 knots? Personally I'm not really sure that I can give you a good answer to that. I'm not very much into boat racing, I rarely need to get from A to B really really fast and I have no urge to show off my 60knotsawesomenesscraft!!!1 (there aren't really many people around here to show off to anyway). So why? I still do like a good thrill as I did 10 years ago in my early youth, but some fine doses of adrenaline can be found even in 20 - 30 knots in the right circumstances and in the right boat.

Obviously, a small boat like a 16-18 gets slightly too small for me as we do have a lot of bad weather here and I often for example need to get to (or from) our cabin on an island in the middle of the fjord. A (purposely designed) 22' performs so much better in rough weather than a "puny" 16,5', but still I could have made due with "only" 30 or 40 knots. Of course a 40 knots march speed is way better than 23 when heading into the longest fjord around here, but still I'm rarely counting minutes when I'm out for transport purposes.

So I guess it comes down to that I actually am nuts. I'm feeling a desire to own and contribute to the creation of a boat that's capable of doing 60 knots on a good day and eating waves for luch in 40 knots on a bad. I still don't feel mature enough to explore the wonders of sailing, but what do I know before I've tried? I know that I love the ocean at whatever speed, but still I'd like the rush of speed and feeling of flying across the sea. Of course it's pricey, but I don't really care that much about the money. Others fancy sportscars or an expensive BMW. I fancy boats. Money comes and goes anyway. It's all about what use I put it to.

Is this a stupid pursuit of interest? Perhaps. But is it worth it? Right now I can't believe anything else. The taste is like the butt; It comes in all kinds of shapes. Perhaps one day I'll find myself drooling on a nice little sail boat, but not today, and most likely not tomorrow either.

My 1.4c

Regards
Ronny

 

If you take your drawings to a local Naval Architect (try the phone book) they'll charge you about the price difference between a 200HP and 250HP engine to refine your hull shape and specify a laminate. As 200HP is actually more than enough for your proposed boat and the final product will work/run better it would be money well spent.

 

I've found some candidates for the refinement and laminate specification, so I'll check out the cost next week. I've considered a 200hp engine (50 hp loss should equal 5-10 knots of speed loss I think, depending on the propeller), but I still think I will have the boat designed for 250, both in case I change my mind and to be on the safe side.

Thanks for all comments.