DRAGONFLY Trimarans User Forum

The ball valve on the head intake through hull needs to be replaced. I'm not certain how to measure the through hull to spec a replacement. It looks like 1" straight pipe thread, but that seems unlikely since everything else on board is metric.

Does anyone here know the size, or know how to determine the size without opening a hole for seawater to get in while I measure?

Through hulls are National Pipe Straight Thread (NPST) also equivalent to British straight pipe (BSP). Look at Groco, Forespar pages among others.

Google NPST and you will find a dimension table. On our boat head used a 3/4 BSP which is slightly over 1" OD.

We replaced ours (all seven) with Groco bronze through hulls with matching Groco BSP valves, rather than the Groco Female BSP adapter flange which has a male pipe thread nipple so you can use a pipe thread valve.

Forespar Marelon units come as a NPST through hull flange with a matching NPST valve.