hard mount or rubber mount?

My recent purchase of a glass/ply twin hull 10mx4.5m to suit for wheelchair access has led to a question about hard mounted twin isuzu diesels 75KW(not verified yet) and shouldn't they be properly rubber mounted?

boat pics http://www.aussiecommunity.com.au/pics/twin-hull/twin.htm


When questioning the previous owners the response was basically they asked around which was better and then decided the easy way was to hard mount them.

Of course when doing the sea trial the noise is horrific to say the least. well noisy to me anyway, and vibrations.

Although take into consideration there is zero insulation anywhere on the boat so the noise will be louder anyway.

they are penta legs 19inch props on them.

anyone know if its common to hard mount engines? I will endeavor to have a mechanic to look at it and get them rubber mounted but wanted to know of any opinions on this.

thanks

russ

 

The rubber mounts are expensive as the fuel system ,exhaust system, cooling system and even the drive may need to be changed to allow for the large movements of working flex mounts.

2 inch 2lb lead sheeting from Soundown shold be first choice , your gona buy it anyway.




FF

 

Ah ok well that might explain why they hard mounted it then.

I have no idea what you mean when you say this?

2 inch 2lb lead sheeting from Soundown shold be first choice , your gona buy it anyway.

lead sheeting to soften the vibration?

 

Lead composite sheeting to SILENCE the NOISE.

A boat that tingles a bit is no problem , but one that assaults your ears is a horror.

FF

 

Wow, that's a sweet project!!

 

Hi Danny, it sure is .

Just had it surveyed in water and is definitely sea worthy. Now I just have to drive it up river 18KM's to the marina from where it is moored to be lifted for antifoul and the legs serviced. It will get slipped 11:30am Tuesday morning.

Then hopefully back in water on to a berth by Thursday so I can start to clean her up a bit. Will talk to eh marine engineers too about the possibility of mounting the engines to reduce vibrations.

 

Ya know, your post now HAS ME wondering- in my last boat, I had an OMC Cobra drive, and the engine had soft mounts in the front, and soft mounts (fat rubber washers) where it mounted to the transom, but then again it did have the drive coupler which essentially is another fat rubber unit I guess that would isolate any vibration to the engine..... My current project has the engine coupled to the velvet drive, and then the two units are mounted to aluminum angle via soft mounts. NOW I'm wondering if being that the v-drive is going to be hard mounted to the stringers, if that will be a problem. I'm thinking that it might be...... uuughhh anyone know if I have to hard mount the engine and velvet drive too now??

Sorry for the hijack.....

 

in my other boat i think the velvet drive is soft mounted http://www.boatdesign.net/forums/showthread.php?t=23470

the engine is stern and is also rubber mounted. But yes they will need alignment. in my case the v-drive had to be raised up higher to get proper alignmeant.

 

Razgo

When one wants to soft mount engines, you need to check what are the 'limiting factors' of movement, like Fast Fred stated. It is no use resilient mounting the engines if all attachements (when are connected somehow/somewhere to your boat's construction) are rigid. Then one should lok where the thrust is taken, in this case with stern drive units I think there will be a rubber ring/gasket to the transom ? Is there a flexible coupling between engine/gear and the stern unit ? If not, forget resilient mounting.

Two other important issues regarding noie and vibration in general on 'smal' boats:

Make sure your mounts or engine feet are placed on top of a dynamically stiff point, so make sure you have a crossing of longitudinals and frames where the mounts/feet go or makes a cross with brackets.
Weight in the engine bed is important too, make it solid and heavy. If it is a hollow tube of some some (engine foundation stringer), fill it with fine dry sand and close it permenantly. Otherwise use solid hardwood as engine bearers.

Second, Engine room insulation is important too, a double working system should be used, like lead/foam type. Lead is not the most favourable material because of enironmental reasons, but works pretty good because of its weight. Weight and soundabsorption are the key words. So, a system of foam/rock/glasswool (whichever you prefer) in at least 1"thick (the thicker the better), a massive layer (weight, like flex rubber sheet) of ~2 mm and then another soft layer of approx. 1" would be a good start. I would prefer using high density Rockwool in 2" faced with reinforced aluminium foil as a cover, 2-3 mm thick massive layer (Bariumsulfate filled rubber 2300 kg/m3) and another 1" Rockwool layer as a total package, fixed with pins and washers.

I know a lot of people use foam, but it degradated quickly and it is not as firesafe as Rock or Glasswool.

BUT, much more important from noise point of view is good closure of the Engine Room, noise is like water, it finds its way, and a hole of 1" will almost cancel out the advantage of insulation in the middle and higher frequencies. So dogg down all hatches (do no rely on gravity for closing) in soft foam rubber, close up all cable/pipe penetrations and make sure you treat as much flat surface in the ER as possible.

I hope this helps....

 

ASM- so basically you're saying that I have to either hard mount everything, or use a flex coupling in between the soft mount engine/velvet drive, such as a cardan joint? Or could I get away with a thicker commercially available rubber washer. I've seen them advertised for used so if you hit an underwater obstruction it will break loose before you start tearing up gears.

 

Yup if you do not have a flexible coupling between stern gear and engine/gear combo, your engine/gear combo will dance around the rather stiff connection of the sterngear to the transom. If I am well informed (I usually deal with much bigger yachts) a transom mounted sterngear takes the thrust from the prop and 'leads' in to the transom construction.

A rubber washer is not flexible enough I think. The whole resilient set up is something delicate and to do it right, all the felxible components must be chosen in conjunction with eachother, or one can face resonance problems.

Normally, when properly engineered, the mounts, flex coupling and bellows should all work in harmony with eachother.

Don't hard mount everything, cables and other attachements to the engine (hydraulics ?!!) can be flexible mounted, but not the engine/gearbox/shaft, that one needs proper attention.

 

Now you got me all confused LOL. Could you let me know if the following would work?

engine and velvet drive (soft mounted)-cardan joint-v drive (hardmounted)-propshaft

 

And you have me confused.. you have a V-drive ? I only remember V-drives where the shaft from engenis goes into rigid remote mounted gearbox. But anyhow, yes the set up you propose should work. Sorry for the confusion, 9 out of 10 times there are no problems with flexible set-ups, but one CAN go horribly wrong, we have seen and measured several of these cases. Do you have somebody to do the engineering for selecting the right mounts ? (like different shore hardness for the gearbox side when needed ?)

 

No, no engineers on my payroll LOL, but I've done a lot of reading and even dug out Glenn L Whitmanns "Inboard Engine Mounting" book I forgot I had. All his pictures show a double cardan joint, but look more tailored towards the go fast boats because they all have long tubes between the tranny and v-drive. Now I gotta find a small cardan joint that fits my flanges- I knew I souldn't have sold the ones I had !!!

 

Here's how my engine v-drive is mounted, except I've flipped the aluminum angles so that I could fit the heat exchanger w/o having to cut the angle bracket.