How do you measure ISP for PHRF?

[FlyingCircus2]

According to yralis.org, ISP is

spinnaker halyard height from the top of the spinnaker halyard sheave to the deck of the centerline

But how does that work with raised cabins? My mast sits on a 'bubble' above the line of the deck that raises the mast about 18 inches. It's easy enough to do the math and add them together.

...but most boats have some sort of raised cabin area. Do I really interpret this measurement literally?

 

[FlyingCircus2]

 

[voodoochile]

 

[FlyingCircus2]

that's a great diagram. What if the mast was the red line - do you have to add the yellow bit?

 

[ryley]

voodo's got it. I and ISP are measured to the sheerline.

 

[ryley]

that's a great diagram. What if the mast was the red line - do you have to add the yellow bit?
View attachment 582873

yes.

 

[Dacron]

Assuming you already have an I measurement documented, you could measure the distance from the jib sheave to the spinnaker sheave and add that to the I measurement? They both have the same deck-level measurement plane.

 

[Meat Wad]

Have your sailmaker do it. That way if it is F'd up, they are at fault.
I measured mine and then I had Ullman measurement and I got a few inches more.

 

[BoksNoRox]

Yes have your sailmaker do it. If he is building a sail for that halyard he will want the correct dimension to get the sail right. If its wrong it is not your problem.

 

[Jono]

It aint that hard.
Tape measure from the fully hoisted halyard to the deck just in front of the tabernacle.
Then get a batten / straight edge and with a person on the other end get the even measurement on each side from the straight edge to the gunnel. Simple. Particularly when you have the boat on the trailer so you can stand there to do it.
Add together three things. Length of halyard clip / knot to start of tape. Tape measurement / Cabin measurement.

 

[Capt Araldite]

voodo's got it. I and ISP are measured to the sheerline.

And don’t forget that the sheerline will probably be a little lower than deck level. Read up on how to determine actual sheerline position.

 

[Somebody Else]

How do you measure ISP for PHRF?​

Use a tape measure.

 

[El Borracho]

Remember that this is PHRF. The committee will only glance at these measurements to determine if the boat essentially matches the class or boats already rated. For ISP they only seek to determine that you have not moved the spin halyard from a fractional to a masthead position. Or the like. Nothing more.

 

[dacapo]

Remember that this is PHRF. The committee will only glance at these measurements to determine if the boat essentially matches the class or boats already rated. For ISP they only seek to determine that you have not moved the spin halyard from a fractional to a masthead position. Or the like. Nothing more.

You are correct. Or see a sail dimension that in no way can fit on a boat with a particular ISP dimension.

 

[neuronz]

You are welcome.

 

[bgytr]

Trig.
b^2 +ISP^2 = h^2

 

[Go Left]

Trig.
b^2 +ISP^2 = h^2
View attachment 583653

But nobody has yet proven Pythagorus's Theorem without using circular logic.

 

[bgytr]

But nobody has yet proven Pythagorus's Theorem without using circular logic.

Maybe, maybe not...
Couple high schoolers:

High school students may have just discovered an 'impossible' proof to the 2,000-year-old Pythagorean theorem

<p>An Impossible Proof Of Pythagoras</p>

In the 2000 years since trigonometry was discovered it's always been assumed th...

meetings.ams.org

 

[ryley]

You are correct. Or see a sail dimension that in no way can fit on a boat with a particular ISP dimension.

no, they won't notice that. ORC didn't notice that on my boat, so there's no way PHRF will.

 

[ryley]

And don’t forget that the sheerline will probably be a little lower than deck level. Read up on how to determine actual sheerline position.

I went down a deep deep rabbit hole starting to read the measurer's manual on US Ailing. eesh.