Cutting a hole in the hull

[doca]

I've decided to fit a new log and depth transducer and have finally faced up to the awful fact that I have to cut a hole in my grp hull. It's a 50mm diam hole and I am contemplating using a good size drill bit first (12mm) and then increasing the hole to the required size. Is there a better and more accurate way to do this ? I then intend to butter on loads of epoxy filler on the inside or maybe just fibreglass arounfd the fitting.

 

[snowleopard]

Buy or borrow a good hole saw. The bimetallic type like this is ideal. Don't go for the DIY type with spring steel blades that slot into a plate.

a small pilot hole, say 3mm will help get it started in the right place. You will need to build up the area around to flat parallel surfaces if the area you are cutting is curved. Abrade well first then use polyester or epoxy filler.

 

[savageseadog]

Yes use a decent hole saw like Starret. If you use a pilot drill it will enable you to drill from the other side, sometimes easier to drill from the outside. Use a slow running drill as the hole saw blade will heat up and blunt.

 

[Lakesailor]

I bought a decent hole saw (Bosch I think) for cutting a hole in my headlining.
You get a mandril to which you can fit different twist drills to with an allen key for the pilot hole and then the size of cutter you need which just screws onto a thread on the mandril. All fits into a 10mm chuck (make sure you get the right size, there are more than one) and it works a treat. I have cut a few holes for access now. Cost about £11 in total.

 

[woody001]

you should drill from inside of the hull, thros making sure you get the correct angle to drill, very difficult from the outside.
-advice from a boat builder.

 

[nedmin]

if I was doing it and i have done it often, do as snow leopard says cheap hacksaw types are useless in anything other than wood.Take your time dont rush when you get nearly thro, go to the otherside and finish .This way you wont chip bits off.

 

[DaveS]

Agree with others on getting a good quality holesaw and the need for building up a flat internal (and, depending on hull shape, perhaps also external) platform. If the hull is thick you may well have to drill to the full depth of the saw on the inside then finish from outside - having a pilot hole the same diameter as the holesaw's pilot drill helps with this a lot since you can "feel" your way in to get the angle right before starting to cut the outside hole. It's not the end of the world if you get a slight misalignment between the holes, since you will want to epoxy the cut surface anyway to ensure that water cannot get into the GRP, and you will have made your holes a bit oversize to allow room for this, won't you? /forums/images/graemlins/smile.gif

You say it's for a "log and depth transducer". Is this a combined unit, or do you need two holes?

 

[roger]

I think we've all assumed that you had no transducer and associated hole before. If you have got a smaller hole already then its reasonably easy to block it up then use the hole saw mentioned elsewhere to cut a larger one.
We have had threads before on filling up unwanted holes with some excellent instructions. You should be able to find them using the search option.
Log calibration will vary according to where you fit the transducer; they like a nice smooth flow of water and you should make sure you can get at the transducer to pull it out for cleaning.
A sounder impeller likes to look down whichever tack you are on which can be difficult unless you have a flat bottomed boat.

 

[nedmin]

try to locate the log so that when you go in reverse the back wash will knock weed of the impeller.Not too near or it may effect your readings.Try and imagine where the prop wash will go when in reverse.it wants clear water in front of it.

 

[sailorman]

site it forward of the keel in "clean" water

 

[doca]

Thanks for all the replies. I am actually replacing a through the hull speed log and an in hull depth sounder which have given up the ghost. The new ones are Tacktick. I was hoping I could use the existing transducers but when I checked today they are not compatible (not "Airmar") therefore a hole in the hull x2 it must be. Like a hole in the head x2 !!

 

[marcusgillies]

Are the new holes in the hull going to be bigger or smaller than the one for the old speed log?

 

[doca]

the new log hole will be larger. the depth transducer will require a new hole as the current one is in-hull.

 

[boatbuilder]

If you have a large hole that you want to make bigger, you have a problem as the mandrell has nothing to drill into. Not so, just stick a piece of ply across the hole so that it covers the centre and use that to support the mandrel, you will be able to cut a larger hole without the cutter skidding across the hull /forums/images/graemlins/cool.gif

 

[cpedw]

I accept that using expensive tools is probably better, but I have successfully made holes for logs in fibreglass using a cheap holesaw. First, use the holesaw to mark out the hole. Just take it a few mm into the fibreglass. Now put the hole saw aside and get a small but adequately long twist drill ( say 2-3 mm diameter). Drill lots and lots of holes in the slot that the holesaw defined previously. You can apply the technique from both sides. The hole saw's pilot drill ensures that they line up. When it's reached a suitably spongey consistency, go back to the hole saw and even a cheap B&Q model will get through the remaining material in no time. With a cheap hole saw, it's even more important to use the hole saw slow as it finds fibreglass very tough and gets awfully hot (and blunt) otherwise.

Derek

 

[snowleopard]

A good quality hole saw from Screwfix will set you back £7 or for £10 you can get a set of 6 blades with two arbors. Once you have them you'll be amazed how often they come in useful! You might even end up with a hull like swiss cheese /forums/images/graemlins/wink.gif

 

[Lakesailor]

Why use a cheap saw when you are dealing with your boat, your safety and possible expense of putting right a bodged job?
For less than £10 you can get a real hole-saw.
See Here for instance, as Snowleopard says

 

[Katouf]

Re: Cutting a hole somewhere else

I needed to cut a hole in my Centaur moulded cabin sole to access the water sloshing about underneath. (I think I've got blocked limber holes somewhere between the chain locker and the engine compartment where the hand pump sucks from). I wanted to fit an inspection hatch such as those fitted to watertight lockers in dinghies.
As the hole could be about 6" diameter, how would you do it?
Alan

 

[duncan]

Re: Cutting a hole somewhere else

not trying to be faecitious but just get a 6" hole saw...........

the lip of the 'hatch' will cover up quite a messy cut, so multipul drilled holes and a saw, or even a jigsaw, will do but 2 minutes for a perfect fit v messing around for an hour wins everytime.

 

[jimbouy]

Re: Cutting a hole somewhere else

Alan,

Is there not an existing inspection hole/hatch? Blue Moonlight has one that looks original.

6 inches sounds a bit big.... Blueys is around 4 inch .... big enough to poke a hose through, which in turn is connected to a small pump.

I also have a hole from the under cabin area to the engine bay. Its just behind the very base of the flywheel and there is a bath plug that bungs it up to stop oily water getting from the engine bay. Equally it has to be removedto drain the under cabin area.


I'd get the biggest hole saw I could and a smaller inspection cover.


Jim