How do I calculate the keel weight for the model Star 45 I’m building?

[Samuel CroweParticipant @samuelcrowe16582]

Hi model sailors,

I'm building my first model boat, a Star 45 with a Braine steering system. However I have no idea how to choose what weight my boat's keel should have. I can post photos if requested.

[Samuel CroweParticipant @samuelcrowe16582]

I’m building my first model boat, a Star 45 with a braine steering system. However I have no idea how to choose what weight my boat’s keel should have.

[Colin BishopModerator @colinbishop34627]

We don't seem to have many yachties on this Forum but there is a lot of online information about Star 45s such as this site: **LINK**

Just Google Star 45 model yacht and you will get lots of hits.

Colinj

[Malcolm FraryParticipant @malcolmfrary95515]

From https://www.theamya.org/boats/star45/Star45_Rules.pdf rule 1.10 says that there is no maximum weight, but the minimum weight is 12 pounds ready to sail (OK its a radio yacht, but the numbers will be close). Once you have an idea what it weighs without the bulb, you know what is needed as a minimum, the maximum is that which does not either sink it or break the fin. More weight will let it handle more wind with the same sails, but due to it sitting lower and getting more hull drag, it will need more wind.

I hope that the chosen pool is access-friendly. Braine boats are fascinating (i.e. tricky) to set up. Predicting where they are going, and how fast, is a real art. Experience says that a self steering yacht will only steer straight when it knows that it has a short sail but you have the longest possible run to get there first. One of the reasons that I put radios in mine.

[Samuel CroweParticipant @samuelcrowe16582]

Thanks for the replies!

I have tried to google it, but it seems there is no basic ratio of ringing to keel bulb weight.

Malcolm Fray, your post was particularly helpful. but 12 pounds minimum? That's 192 oz, heavy! I intend to use a lead fishing weight (due to perfect shape) for a keel bulb. Perhaps I should just try out different weights for a medium to light wind. I have some experience with braine steering, it just seems so much more work, expense and difficulty to put batteries and a servo in it.

Edited By Samuel Crowe on 16/10/2017 23:53:12

Edited By Samuel Crowe on 16/10/2017 23:53:37

[Paul TParticipant @pault84577]

Hello Samuel

This document from across the pond might help Link you will find that our american cousins are quite exacting regarding the specifications of this particular sailing model including, as Malcolm pointed out, a minimum weight requirement of 12lbs (5443.11g)

Paul

[Ray Wood 3Participant @raywood3]

Hi Samuel

I'd estimate 7-8lbs seems average for a 50"Marblehead, the Star is not much shorter but being single chine has a fair beam/displacement. You can take some weight off the lead bulb if she comes out over weight or too deep in the water.

Regards Ray

[Samuel CroweParticipant @samuelcrowe16582]

Hello,

Thanks again for the fast replies!

I understand the American weight requirement, however as I was using rough plans, I did not scale my boat properly (it is my first ever). I've figured out that since the length of the proper Star 45's is 45 inches, and my hull is 28 inches long its a multiple of roughly 1.6. So since the keel bulb minimum weight is 12 pounds, 12 divided by 1.6 = 7.5 pounds for my boat. That is still very heavy and about $8 British pounds worth of lead. It just feels like the boat needs something smaller. Any other thoughts?

Sincerely, Samuel

[Malcolm FraryParticipant @malcolmfrary95515]

Posted by Samuel Crowe on 18/10/2017 02:01:13:

Hello,

Thanks again for the fast replies!

I understand the American weight requirement, however as I was using rough plans, I did not scale my boat properly (it is my first ever). I've figured out that since the length of the proper Star 45's is 45 inches, and my hull is 28 inches long its a multiple of roughly 1.6. So since the keel bulb minimum weight is 12 pounds, 12 divided by 1.6 = 7.5 pounds for my boat. That is still very heavy and about $8 British pounds worth of lead. It just feels like the boat needs something smaller. Any other thoughts?

Sincerely, Samuel

For the full size model according to the information in the link, it is not the bulb weight at minimum 12 pounds, that is the entire boat including the keel weight where 12 pounds is the minimum sailing weight. Since your model is about half the length, and when talking weight you are also talking volume, thats about 1/8th the weight. With a ballast ratio of about 50% you are in the 24 oz area total, so a keel weight of about 12oz, maybe 14oz. This fits well with the old Akela design (used to be a free plan years ago) that was 25" long and carried a 12oz weight as well as the modern 25 inch racers (DF65) which carry the same weight.

Shrunken yachts rarely perform up to expectations without there being some modification, usually a longer fin with the same weight to enable sailing in a wider range of conditions. The bulb does need to be a very good hydrodynamic shape, casual lumps cripple performance.

[Samuel CroweParticipant @samuelcrowe16582]

Hi,

You have my many thanks Malcolm. That has been one of the most useful replies so far. What you said before makes sense now. I'll look for something around the 10oz to 15oz range when I do the experimenting for the best keel weight. I'm feeling pretty excited, my boat is coming together nicely. You people that have been in the business far a while would probably cringe if you saw it, but its my first and I'm happy with it!

This forum has been massively helpful to me, my sincere thanks to all who contributed!

Samuel

[Malcolm FraryParticipant @malcolmfrary95515]

Just looked at my Victoria. Its 30" long, weighs 5 lb and almost half that weight is the bulb. The draught is about 9" (waterline to bottom of weight, and it carries 400 square inches of sail on a 39" mast. While by no means the same boat, yours is somewhere between that and the old 25" Akela , now known as "XL25" with drawings and more information at http://www.sealevel.demon.nl/english/downloadsXL25modelyacht.htm

[TelstarParticipant @telstar]

Hi Samuel

Try this site **LINK** It may offer more confusion or maybe not. Success with your build

Cheers Tom

[Tim RoweParticipant @timrowe83142]

Hello Samuel
​Lots of good information coming your way particularly around the scaling effect that does not work in your favour. Malcolm was certainly pointing you in the right direction and I am sure he won't mind me pointing out that when he described making the keel longer he really meant deeper. On a yacht, long keel has a meaning of its own and the important thing is to get the keel weight (and therefore the centre of gravity) as deep as possible.
​As to the weight required on your model it is very easy to find out by practical testing in a bath. On the plan it will show the designed waterline and you can mark this on the hull. You now add weight making an allowance for mast, sails, fittings, radio, batteries etc and when the model is floating on that waterline you can measure the added weight and that is the amount of lead you have to add to the keel. The link from Telstar is a very useful way of getting the dimensions of a bulb shape that is the weight you want. Lead is not very strong so although a narrow shape creates less drag it will get bent easily if it overhangs the keel. A model yacht for just pottering about will be fine with something a bit stubbier.
​Finally and due to the scale effect and not having to comply with any rule I would keep the depth of the keel only a bit less than the original and not in proportion. Remember, the wind doesn't know what size the boat is.
​As to ways of casting the lead you will find lots of ideas (and precautions) on U Tube.

[Ray Wood 3Participant @raywood3]

Hi Samuel

I the quest for your keel weight for your Star 28 , you may consider Vic Smeeds Panache design 30" long has a keel weight built on the lower part of the fin with lead sheet flashing and interestingly for you he quotes 2 1/2lbs for RC and 3 1/4lbs for free sailing which fits the bill for you with your vane control, whilst googling I also found the plan in pdf format so I might have a go at building one Vic was and still is the Boss and gets my full respect.

Regards Ray

[Andy StonemanParticipant @andystoneman15177]

Hi Ray, what sort of keel weight do you have on your Thames sailing barge ? The barge I've recently built has a 3kg bulb weight but I'm thinking its not enough should the wind pickup, its a 1:24th scale. Also whats the depth of your fin/keel ? I took it for its first run the other day and was a bit disappointed on its performance, Most probably due to the lack of wind, 11klm/hr.

Regards, Andy

[Ray Wood 3Participant @raywood3]

Hi Andy

My barge has a winged keel moulded in GRP with 6 1/2 lbs of lead poured into the wing, its about 9" deep, she sails better with it than the much deeper Marblehead fin keel, and is much easier to launch. It takes quite a lot of tuning of the rig to get competitive boat speed, I've been sailing Veronica for 3 seasons now and I'm just getting there, but I have more plans for next year. Make sure your hatches are sealed, they do tend to beat to windward with wet decks in a blow

Regards Ray

[Andy StonemanParticipant @andystoneman15177]

Hi Ray

So i`m probably not too far from the correct weight of 3kg, the allup weight of my barge is 9.7kg the fin is a traditional keel shape with depth of 9-1/2" so I'm not too far from the right depth, Your winged keel looks interesting. My barge is scratch built and a lot of guessing from photo`s of other barges plus. More trials underway hopefully next week when the winds are more favourable.I liked your H.M.S. Diamond model, an old ship of mine.

Regards, Andy

[Ray Wood 3Participant @raywood3]

Hi Andy

Not wanting to Hijack Samuels thread, I've started a Thames Sailing Barge thread in the Sailing section.

Regards Ray

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