BRe windward performance

[Michael Rogers]

Peter's last sentence reminds me of the old chestnut. A small boat, containing a heavy anchor, in a tank of water: the anchor is thrown overboard: does the water level in the tank rise or fall? [This has absolutely nothing (I think) to do with BRe windward performance.]

[Michael Rogers]

Sorry, that should be 'last but one sentence'.

[Peter Taylor]

We filled the tanks and repeated the test under roughly the same conditions, a marginal improvement was achieved with the boat being flatter, and the conversation moved on to the weight of the water in the tanks and boat weight/reduction in speed, does the boat weigh more when afloat with the tanks full.

As to the last question the answer is definitely "yes" and I can only remind people of my library article...
http://www.swallowyachtsassociation.org/?page_id=1183 !

I use Seatern's track on the chartplotter to determine tacking angle but, of course that only works in terms of boat performance if there is no current. In the latter case I think I can just about achieve 90 degrees OK but I'm not sure that gives me the best velocity made good, Seatern goes faster through the water if I free off say 3 degrees or so (which I can measure using the Tiller Pilot to steer). Whether the speed improvement is enough to improve my VMG I don't know but I suspect it is.

As to Michaels chestnut, a hint - in the boat the anchor displaces it's own weight of water. When the anchor is at the bottom of the tank it displaces it's own volume of water. The anchor is denser than water (assuming it sinks when thrown overboard!). I'll leave people to work out the rest!

Peter

[BobT]

I suspect there must be an explanation somewhere on the forum but I can't find it. What part of the boat do I put my spirit level on to get the boat level so that I can then assess the mask rake?
Cheers to all,
Bob

[Peter Cockerton]

Bob

I pondered this one some years ago, if you have a smartphone you can use a clinometer app, you the hold the phone/iPad against the surface of the mast and it will display the upright angle for you. Hull level does not have to be considered with this method of measurement from my understanding.

https://www.youtube.com/watch?v=4mJh94Cm3zw

I guess you can prove the theory by taking the mast upright reading and then (assuming the boat is on the trailer) lowering/raising the jockey wheel to change the boat level and take the reading on the mast again.

Peter C

[BobT]

Hi Peter,
Had a look at the app but not clear how that helps find/set mast rake. If my understanding is right (no guarantee there) then mast rake is relative to some "horizontal" part of the boat and this part of the boat needs to be set horizontal before one can use inclinometer?
Could you or others clarify?
Cheers
Bob

[Peter Cockerton]

Bob

“If my understanding is right (no guarantee there) then mast rake is relative to some "horizontal" part of the boat and this part of the boat needs to be set horizontal before one can use inclinometer?”

Bob, this is not my understanding, the app and device works independent of the boat horizontal plane position. Basically if the iPad for instance is held vertical the reading will be 0 degrees, if the iPad body is placed against the mast the reading will reflect the vertical upright degree of the mast.

As is said in the previous post if this is in doubt take the current mast reading and assuming the boat is on the trailer alter the boat horizontal position by adjusting the jockey wheel and take the mast reading again.

Peter C

[johnguy]

I'm like Bob T here, rake relative to what?

 I think it is relative to the waterline as it floats. So have the boat in the water and use the inclinometer then to check mast not leaning forward.

If you are on the trailer you can make the inclinometer read whatever you like by adjusting the jockey wheel. That tells you nothing.

[Graham W]

If you have a waterline stripe painted on your boat and it’s accurate (mine is), you can use that, the inclinometer and the jockey wheel to get the boat absolutely level on your trailer.  Then check mast rake as Peter suggests.

An alternative quick and dirty test of mast rake was mentioned by Ray S in this thread last month - attend a Swallow rally and compare yourself against other boats when rafted up.  Even small variations in rake are fairly easy to spot and big variations really stand out.

[johnguy]

I checked mine on the water yesterday with the smartphone and it was exactly vertical. So some shroud tightening to come next week. I also had a Graham look along the berths and it is noticeable that a vertical mast is unusual, most are tipped back a little bit. I think mine has come more vertical since I put some effort into tightening jib halyard trying to get better on the wind. No affect I can feel on helm loading though, boat is beautifully balanced.

[Peter Taylor]

Bob, this is not my understanding, the app and device works independent of the boat horizontal plane position. Basically if the iPad for instance is held vertical the reading will be 0 degrees, if the iPad body is placed against the mast the reading will reflect the vertical upright degree of the mast.

But as johnguy says, if the boat is not floating you need to know the angle of the hull to it's normal alignment when afloat. I use the table in the cabin (fixed to the top of the centreboard case) as an indication of the designed waterline since I assume matt would have intended that to be horizontal - and when afloat it is. So using the inclinometer built into the smart phone I measure both the rake of the mast and the slope of the table and hence work out the mast rake when afloat. Alternatively i just wait for the tide to come in!

Peter

[johnguy]

About the mast rake and what it is referenced to. I had a chat with Nick Peters, the Swallow technical director, at the boat show. He confirmed that the rake is relative to the waterline. So pop the boat in the oggin and pop an inclinometer on the mast. If you don't like the rake, you can adjust the shrouds or ask the mother in law to sit further aft. Nick also emphasised that in practice the rake makes very little if any difference. he says a bit of aft rake helps the boat look good in photos, but doesn't really affect sailing performance.

[jonno]

Too rakish?

[Sea Simon]

Too rakish?

I had to full size the pic to see that the mast is on the RIGHT of the pic!

May seem daft, but my racing dinghy is factory fitted with a mast raking system, primarily so as to reduce weather helm, upwind in a strong blow. Hence increases speed too. It works....but its not a BRe....

Ever seen a Contender in full flight? Rig rake looks very similar.....