pod technicalities

[MICK H]

Hello all i am new to this forum and have been reading lots of threads on boat repairs, all very informative.What i would like to know is i'm going to put a pod on a 15 foot cruise craft rapide and run it with a 75 -85 hp motor, i understand that for every foot i go back i have to raise engine height 1 or 2 inches {please clarify}. my questions are 1. how long should i make the pod {450 mm -600 mm}
2,the pod is going to be extended from the hull ,at the back of the pod where the engine mounts, how high from the bottom of the pod should i mount the outboard , and 3. i'm going to make the pod myself from a mould of my own making, should i use marine ply inside the fibreglass for the engine mount or could it be made from core matt.?.aany help would be much appreciated.
forgot to mention i want to run a 25inch leg .
thanx....Mick

[MICK H]

This is the style of pod i'm going to build, ;D

[jimbo59]

Gday mick ,i don't really like the look of the angle that the water takes on after coming out from under the hull in that photo,(to much dirty water)Another way is to box up the back of the transom and glass it straight to the bum,just continue the original shape of the hull and make it a 25mm longer than full tilt angle.

cheers...jim

[dnej]

I agree,go as an extension to the bottom.The pods can be bolted on, as they do with the VMR cats.Take a look at my pods.The transom is 25 inch,and the most important thing apparently,is the angle of the new stern, created by the pod.

Also, make sure you dont build in a banana shape, as this causes porpoising.My motor, sits the same height as normal.
David

[BM]

Mick,

One inch up for every 12 inches aft. This is the general rule of thumb.

1) Pod length is usally worked out on the basis of the engine being able to trim up fully. Typically 600mm will achieve this (probably even 550mm will do). This will of course be altered if you incorporate a mini splashwell in the transom.

2) Your engine mounting height will be determined by the height you set the pod at. EG: when I fit non full depth pods I set them as low as I can get them. This then requires an alloy plate (I use 25mm thick) which is sandwiched between the engine and the aft end of the pod.

3) Either marine, or structural ply (both have the waterproof glue and will be suitable) on the engine mounting face. I would not use core mat as I don't believe it has the strength.

Cheers

[Kerry]

BM, 25mm alloy plate ???? bit of an overkill isn't it, unless your hanging 1000hp off the thing??

[BM]

Sorry Big Kez :)

I should have said, if I am fitting a V6 to the pod I use a 25mm alloy plate. Smaller motors a 20mm plate will do (or less).

Cheers

[MICK H]

Thanks for your replies fellas and sorry i meant to say i'm going to make it an extension of the hull, idon't like the step up underneath either. I'm going to take a skin off the bottom of the boat at the back so i can get the exact shape. It will then bolt on from the inside of the pod and i will glass the boat to pod join for a perfect fit. It is mainly the dimensions i'm not exactly sure about and final hight of the motor mount .

[FNQCairns]

Mick the motor mount if you are doing a planing suface extension form of pod(good idea on your boat) can be viewed as simply the same mounting measurements as your current setup. With no lift (setback) nothing really changes (unless you want it to!:).

cheers fnq

[MICK H]

Hi fnq is that the same if you use a longer leg eg: was 20 inch i want 25 inch leg. Should the transom or mount height be any different for a longer leg?

[BM]

You will need to lift the lip on your engine mount area by about 3 inches (if the present setup is to suit a standard 20inch long shaft) if you plan to fit a 25 inch engine. Otherwise you will not be able to raise the engine high enough to set your cav plate at the right height. By raising the upper edge of your mounting surface by 3 inches it gives you scope to play with (up and down) to get the perfect engine height.

Cheers

[MICK H]

Thanx BM ......cheers mate.

[SO70]

if you do your calculations right there is no need to mount the engine to an aluminium plate.
a full depth pod is virtually a hull extension which means more wetted surface area, more weight and more horsepower.
a non full depth pod means slightly higher engine mounting, propeller running in less turbulent water and less gearbox/skeg drag.

[FNQCairns]

25 inch leg, I am in the same boat, for eveything just add 5 inches and consider it the same, the extra height is nice to keep the powerhead away from the slop at the back.

cheers fnq

[Noelm]

a bit of confusion here, if the pod is just an extension of the bottom of the hull then the motor height does NOT change and the shaft length does not change the mounting height either (just the pod 'top" is 5 inches higher) the cav plate is still at the surface, BUT, if the pod is NOT an extension of the true bottom but has a large step up from the bottom, then the "up and back" comes into play, I posted a picture of this type of setup once before, so do not be confused, you van make an extension of the hull 10 feet long (I know you would not do so) and the motor height will still be the same.

[BM]

Noel, He was talking about going to a 25 inch engine, and if that hull is set up for a 20 inch engine he will need to jack the engine up higher. No confusion there at all. Nothing to do with pods. Only transom heights. Take any boat with a 20 inch transom, you cannot mount a 25 inch engine directly to the transom, the trunk is too long.

SO70, its not about calculations mate. The purpose of the plate is to allow the pod to be fitted as close to possible to the bottom of the hull to minimise any canti lever effect caused by a stepped pod sitting 8 inches from the bottom of the hull.

But to each his own. I'll stick to fititng pods to my customers boats in a manner that I know works.

[Noelm]

hhhmmm I must have misread/interprted what was being stated, I thought he was making some new pods, and as such he can make them to suit any shaft length, and I agree about the 'top lip" height (that is what I said, I thought) what I was trying to relay is that regardless of shaft length, the cav plate remains in the same running position, a lot of people think that if you go to a 25" shaft, you have the propellor 5" lower in the water (if you get what I mean) and that is not the case, it is very hard to put into text your "minds eye" and can be confusing.

[BM]

Agreed. I just re-read it all. His concern was the right height to set the mounting area.

I would not have thought many people would think the gearcase is 5 inches lower in the water with a 25inch shaft? Or perhaps they do?? I guess we take it for granted sometimes that others understand what we understand?

Cheers

[MICK H]

Thanks fellas i understand what you are both saying {same thing different piece of string};) . I'm making a mould off the back section of the hull so the pod bottom is an exact extension of the hull.I will use this section to make a dummy pod first so everything is spot on and then i'll take a mould from that, and that should just about do it.This might sound like a lot of bother but i use fibreglass and a lot of apoxy and stuff like that for work. A lot of materials i already have so i may as well use them 8-) . Is formwork ply ok to use to fill in the transom ?
Thanks again......Mick

[FNQCairns]

NO the phenol covering doesn't let the fibreglass get a grip and it will soon enough de-laminate, CD grade ply is OK or sand the phenol black off your form ply.
Be sure when you drill the engine mount bolt holes you drill oversize fill with epoxy then redrill correctly for you mount bolts. Some glue with epoxy a piece of bolt sized conduit or SS tube to help with crush if a less solid structure is used like foam or core material.

cheers fnq

[BM]

Be sure when you drill the engine mount bolt holes you drill oversize fill with epoxy then redrill correctly for you mount bolts. Some glue with epoxy a piece of bolt sized conduit or SS tube to help with crush if a less solid structure is used like foam or core material.

cheers fnq

I am intrigued by this most unusual approach!! Please explain your logic.

Cheers

[FNQCairns]

Hi BM depending on what structural component is used within the pod (can be solid FG) if some of the more exotic materials are used (relative to wood) they can have a tendency to crush so a crush tube set sensibly shorter then the total thickness can be used, mostly epoxy is the glue of choice.

If wood is used depending on what the builder decides he may also use a crush tube but filling and drilling does away very nicely with the opportunity for water logging.

As you would know, every step is dependent on what construction is going to be used.

Mick if it were me I would use CD grade ply, iso or vinyl ester as the resin, and use chop strand only as the separating layer between multi axial glass, can be very strong for thickness/weight/time/cost than an equivalent predominantly chop strand layup and let it cure for 2 weeks.

Cheers fnq

[BM]

Fair enough!! You have your reasons and have explained them well.

However, if you placed an alloy bar (say 50mm high, by the width of the clamp bracket area and say 10m thick) drilled to the bolt hole pattern, so 2 bars, 1 for the top bolts and the other for the bottom bolts, then compression of the timber would be minimal given the surface area provided by the bars.

Although, even that extent is (in my opinion) excessive and some large flat round washers inside the pod are suitable for the task.

Look at most new boats, bolt heads and washers on the inside of the transom. Sure they will pull in over time. So perhaps we should fit a plate that mimicks the clamp bracket and then attach the nuts to the bolts? This will certainly spread the load a lot better but is it warranted?

Definitely not knocking your approach. Questioning it YES, knocking it NO. We all do our things a little differently... :)

[SO70]

Mick,

One inch up for every 12 inches aft. This is the general rule of thumb.

1) Pod length is usally worked out on the basis of the engine being able to trim up fully. Typically 600mm will achieve this (probably even 550mm will do). This will of course be altered if you incorporate a mini splashwell in the transom.

2) Your engine mounting height will be determined by the height you set the pod at. EG: when I fit non full depth pods I set them as low as I can get them. This then requires an alloy plate (I use 25mm thick) which is sandwiched between the engine and the aft end of the pod.

3) Either marine, or structural ply (both have the waterproof glue and will be suitable) on the engine mounting face. I would not use core mat as I don't believe it has the strength.

Cheers


BM, calculations do come into play when playing with non full depth pods.
if you do the calculation right there is no need for the aluminium plate between the back of the pod and the outboard mounting face.

[BM]

To each his own SO70.....

Sure, if your only consideration is the height of the pod then yes you are correct. No plate is required.

However, if boat performance when fitted with a non full depth pod is more important (it is to me and to most people) then pod position is paramount and engine location is alterable.

But hey, whatever works for you is fine......... Not saying you have to accept my explanation. In fact I don't care!!! I know what works for my customers boats!! Thats all that counts.

Cheers :)

[MICK H]

This is the reason i started this thread, everyone has an opinion on what is best as far as pod construction, type and placement.I think it helps to hear everyones opionions, i can then make a more informed choice. Thanks all for your input.
If anyone else has anything they think might help then feel free............Mick