Building a single scull

Hi Guys.
I am building a single scull and i have never worked with fibreglass before and had some questions.
my boat is going to be 8m long and 275mm wide and i am making a plug using 40 precicely cut ribs seperated 200mm each.
For my mould i was going to use tooling gelcoat and one layer of tissue and two layers of csm with wooden reinforcement to reduce flex. I was wandering if using polyester resin for the mould would be ok with tooling gelcoat and be strong enough for the mould.(the mould will have plenty of reinforcement)

when i come to building the boat from the mould i am definitly going to use epoxy resin. i planned to use one layer of tissue and 2 layers of 200gm cloth.
this type of shell is meant to be stiff and light. they are extreamly fragile by nature and a good carbon boat weighs around 14kg and a fibreglass one around 17.
i was wanting to know if 2 layers of 200g cloth would be strong enough.
the boat will have a large stringer down the middle and several ribs hopfuly keeping the boat stiff.

is gelcoat suitable for epoxy and polyester resins?

approximatly how much epoxy should i buy for the boat?

will 2 layers of 200g cloth be strong enought so stop inward pressure breaking the boat?

is it safe to use polyester for the mould?

any opinions please.
thanks
Max

 

Max
No. You will need at least three with 200gsm cloth and regular bulkheads. The lightest is to use two layers with a core. Make a test piece and load it to see what I mean. The deck really needs to have a core. Even with three layers it should come in under 14kg if you vacuum bag it.

If you want the strongest, lightest and fastest hull that is easy to build go for a hard chine hull. If you plan to build a lot of boats from the mould then the mould could have merit.

I can do the comparative performance analysis based on your weight and power but I expect the hard chine will be equal to, or even outperform, the rounded chine hull. There is a lot of misinformation about the benefit of lowest wetted surface of a rounded chine hull compared to a hard chine hull. With these boats the wave drag has a big bearing on the shape and a hard chine has merit over the round chine in this regard.

I have linked two of my examples. One is the lowest drag hull for 75kg person and power level of 150W designed for 12kph. It was formed over a foam plug with round chine and is 7.2m long. The other is designed for easy building designed for slightly lower power level and is the lowest drag hard chine hull for 11kph with 70kg person plus boat. It is 6m long.

The hull on the second one is 200gsm CF over 3mm Klegecell and the central hull weighed 11kg when painted. The whole boat with the drive and outriggers is 20kg. It was much easier to build and is an all-round better hull.

This is the 7.2m one:
http://www.adventuresofgreg.com/HPB/uploaded_images/P9070158-720160.JPG

This is the 6m one:
http://www.boatdesign.net/gallery/showphoto.php/photo/15138/size/big/cat//ppuser/18624

The hard chine will likely have slightly better stability as well.

The beauty of the hard chine is that it can be built from preformed flat panels. These can be produced with good control over the variables so the end up very consistent.

Rick W

 

Thanks Rick
I think i will stay with my original design. The design is pretty mutch a copy of the fisa designed hull which is pretty mutch all out speed and i have started the plug. 14kg is actually the minimum weight alloud to race in moast regatters. so if it is possible to get the weight close to that and still have reasonable stiffness that would be awsome. if i used 4 layers of 200gsm cloth 8 metres long and 0.5 metres wide (way way more than the boat needs, i dont know its surface area) that would only be 3.2kg of glass. i dont know how much resin is used and i dont know how much it weighs but that allows for a lot of resin.

 

It is probably worth using a 6mm core for the deck. It will improve the overall rigidity. You will find 3 layers will flex quite easily when flat. It is more rigid in the curve of the hull. You will need bulkheads below the loaded area as well to spread load into the hull.

It is worth the trouble to make test layups. You get much more value for your CF if you have a separating core. Also you can determine the amount of epoxy you will need for the eventual layup.

With CF you should be able to come in under 14kg if you take care.

Rick W

 

"the hard chine will be equal to, or even outperform, the rounded chine hull."
Rick, you've probably already covered this in other threads but do you have empirical evidence to back this for a skull?

 

As a rough rule of thumb, you can always calculate the weight of resin equal to the weight of fibre. So, in your case 200gram m² per layer. Of course a very professional done layup can be made a bit lighter.
The hint with the core was not the worst one! Though I would prefer a 1,5mm Aramid Paper Honeycomb in your case (it is easier to get a good bond than with foam).

Regards
Richard

 

thanks i think i will use a core but i am having trouble finding one in New Zealand. do any of you know about 2mm-5mm Matiline coremat or 3mm-22mm parabeam.
thanks
Max

 

The two boats I linked to earlier have been used for extensive comparison. The drag of either hull is similar to a rowing scull. I have a yellow round chine hull that is identical shape to the black one.

I have a lot of comparative data similar to the attached that shows the engine bpm and the boat speed on the same plot. I have a large amount of this data that I compare. The best data for comparisons is obtained on calm days.

There are slight performance differences in the hulls. I am getting a slightly higher average with the hard chine.

Both hull shapes were generated using GODZILLA. If they were the same weight then the round chine would be 1 to 2% better at 12kph. However being shorter and easier to build strong the hard chine is also lighter. This brings it slightly ahead.

The minimum weight limit for a competitive scull would penalise the hard chine because you could make a suitable hard chine hull that weighed around 10kg. I am not bound by rules. For the one race I do each year I am in the unrestricted class and anything goes including pedal power. The only thing excluded is rowing sculls because they take up too much room and have trouble seeing where they are going - there are no lanes on a 404km river course.

For a scull I would need to do the comparison for the particular individual and their ability but my expectation is that the performance would be very close. At higher speed the hard chine hull is giving me a bit of lift and is easier to get above 15kph but the prop sucks air because the boat is lifting and I cannot get much over 16kph. With the round chine there is a bit of sinkage at these speeds and no aeration problem. I can nudge 18kph. One day I will set the prop lower on the hard chine hull to see what I can get but till the end of the year I am doing long distance training around the 10 to 11kph.

Rick W

 

Hey i calculated my boats surface area and it is about 2.8m2 ( i gave a little)
so with four layers for 200g glass and resin the hull would weigh 4.5kg and proberly need lots of support. Or one layer of 2mm coremat betwene 2 layers of 200g glass would weigh 7.8 kg and still need some support. have any of you used this type of core before? will using it around the whole hull make it much stronger? those weights are just the hull no deck or bulkheads or other support.

 

That is close to the wetted surface. The entire hull will be 4 to 5Sq.m depending on how much reserve buoyancy you need.

Rick W

 

no i dont think so. if i use a piece of cloth 0.5m x 8m (surface area 4m2) i need to cut of lots of glass.

 

Glass has about 1/7th the elastic modulus of CF for the same weight - it can be even lower if high modulus CF fibre is used. However four layers of 200gsm glass is OK because you are now getting quite thick. There is a lot of added weight to get the thickness though. I expect you will be well over 14kg with a finished boat.

200gsm CF ether side of 3mm foam will be adequate. From a gross strength perspective regarding the hull flexing (hogging) one layer of cloth is enough but would not have the localised panel rigidity.

If you want to easily get to the 14kg weight you should certainly use a core.

Using glass with a core you will need a much heavier layup to get equivalent panel stiffness. Even 400gsm glass on core will not have the rigidity of 200gsm CF.

Rick W

 

Including the deck?

Rick W

 

yea i wish i had more money but i cant afford $750 of carbon and the boat will proberly only be used for training anyway.
excluding the deck

 

Yes - a light one will set you back $10k. Funny how it costs more for less.

For my stuff the weight is a factor in ease of handling so I use CF in everything apart from a few struts and mechanical components. After using CF I find it hard to consider glass. The latter is like rubber by comparison.

It pays to shop around with CF. A friend in the states told me he can get 60" wide CF for USD24/yd. I have seen it advertised in Japan for similar prices but in quantity.

For these sorts of hulls, in conjunction with foam, it is hard to better for low weight and high rigidity.

When you do weight calculations it is difficult to know what to allow for. Even painting will add 1kg and that is lighter than a gel coat.

Rick W