Here is a little article I wrote when a fellow shrank one of my Goat Island Skiffs.
He did actually ask a number of people whether it would be OK to do – and they had all read on the internet that it would be OK.
The Goat Island Skiff (GIS) is probably my most popular plan. Simple building. The light weight makes it easy to handle on shore and provides performance to sail rings around most character boats. It also has a great capacity for people and gear, easy to rig.
A paragon of virtues.
A little while ago I received this email…
Took the boat out over the weekend and raised the sail for the first time. Ignoring the high wind and the inexperienced crew, I was still very concerned with the instability once on the water, in fact we flipped as soon as we began our first tack and thereafter struggled to right the boat again. I was hoping to get your advice on some of my thoughts.
I believe the instability is increased because I have used heavy gauge aluminium tubing for the spars which has the effect of making the boat very top heavy. I will be testing some extremely light weight spars in the coming weeks (possibly bamboo lug and boom).
I also think that the boat sits very high in the water which may require additional ballast to improve stability. I have three questions regarding the ballast:
1. Does the design call for ballast?
2. What are the best materials for consideration as ballast?
3. What are the best locations for ballast placement?
I was a bit confused at this point as no other GIS has ever needed ballast to sail properly – so there was something strange happening.
I answered the email telling the customer that the weight of the spars is part of the issue, but excessive stiffness of the heavy aluminium tubing has a far worse effect. The wooden spars on the GIS are designed to flex just the right amount in a gust so the boat accelerates smoothly rather than staggers and falls over – just the problem the owner was having.
I also asked if there had been any other changes made.
There is one other thing that I may not have mentioned to date in our correspondence.
That is the fact that due to a number of factors such as availability of shed space and availability of ply sheets, I made the decision to reduce the overall length of the boat from 4.7m to 4m (over 2ft shorter – Ed). I made this decision in consultation with Mr xxxxxxxx about the practicalities and consequences of my actions.
I am certain you would not approve of my decision and in hindsight I would have done things differently but unfortunately things are as they are.
It is risky to reduce length.
This is despite common statements on the net that it is “OK to stretch or shrink a design xx%”.
It is a stupid statement. See my response here about “Rules of Thumb for Boat Design“. Many of them are misleading, misquoted or misunderstood.
Generally the builder will have no idea where the boat design is in the spectrum of design. A designer can choose to put a boat in the corner of the design envelope. In this case the Goat Island Skiff gets stability from her high topsides rather than having a wide bottom. There were also decisions made that make sure the boat is nice to steer at any angle of heel.
So in a sense, the Goat is designed into a corner. Which makes sense, because then you end up with something very different from the mainstream – a fast light boat, that is relatively forgiving and even can give more specialist rowboats a run for their money in oar mode. The original “small is beautiful”
So when a boat is designed into a corner like this, it can be altered in some directions … making it more mainstream … which is relatively safe (but oh, so boring!!) or push it outside the envelope of possible design solutions.
That’s what had happened here. Making it shorter was a critical mistake. This decision had:-
- Cut the stability of the boat in proportion to the length reduction – about 15%
- Made the original sail area too large for the shorter boat – by around 22%
- Put the sail a good old English foot out of position relative to the centreboard making the boat difficult to steer and terminating any chance of good sailing performance.
I suggested strongly at this point that I be given the opportunity to work out some way of making the boat work OK using as many of the existing components as possible. So I asked for more information about the exact measurements of the sail that was made.
I have attached some dimensions that I have measured off the boat last weekend. Also find attached the amended sail plans as recommended by the sail maker.
So again … some bad advice for no good reason.
The sail area as recommended by the sailmaker was about the same size as the original sail – but the geometry was a bit different with the centre of effort moving away from the area that is partially responsible for the Goat having a nice steering response at any angle of heel.
The sailmaker had not attempted to reduce the sail area to suit the shorter boat or attempt to get the sail area in the right place relative to the centreboard.
Amendments – or “mending”
The shorter boat only needed around 80 square feet of sail compared to the original 105.
So I took the information from the client worked out that if the sail was reefed at 550mm from the bottom that the centres and area would all work out OK. So an extra reef point fixes that problem and allows the sail to remain full size if it ever is needed to go onto a bigger boat.
Also I drew up some new timber spars for the boom and yard to restore the missing flexibility and gust response.
I saw the owner was trying to sell the boat for around three years.
The rule is – if you want to change the boat – please have a word with the designer first – after all they are the one who understands the boat best. This is doubly true if it is a sailing boat where there are many important inter- relationships.