Hull modifications

[MKP]

Nope, I did not have to kill anyone. Just paid handsomely for it.

We did not expect the gains that we got. I am VERY happy with the results. I do not have specific fuel consumption data prior to the modification, but I am sure I am much more efficient now.

We fabricated a support for the upper rudder shaft bearing with fiberglass over 3/4" marine ply. The platform is glassed to the transom as well as each stringer. I have plans to stiffen it up a bit with some supports on the foward end. I've put it through full-ahead and hard-over maneuvers and it has all held well.

The downhill handling is about the same. When in a large seaway on the quarter the boat tends to veer towards the same side as the swell. I believe this to be related to the fine entry of the bow. The new rudders which were moved further aft are much more efficient at handling the boat in this condition. So to answer your question. Yes the handling is better. There is more rudder response. However the boat is still a handful in a larger following sea. Sometimes to the point where the autopilot will not hold a course. -but all still better than previous setup.

New engines will be ordered within the next few weeks. QSB 305s paired with ZF tranny will be installed this winter adding 100 hp to the scenario. Might see upwards of 24kts now.

[MKP]

You can sort of see the new bearing support is glassed to an previous platform that is glass to about 4 feet of transom. should be plenty of transverse support.

[Guest625101138]

The glassing to the transom solves the transverse restraint. I could not see that important detail.

Looking at your speed versus rpm there is a lot of power to get from 6 to 10kts so I guess things start to settle out after 10kts. Fuel consumption (nmpg) is almost constant from about 10 to 15kts so your target cruise speed of 15kts should be economic.

Rick W

[MKP]

14-15kts seems to be the sweet spot.

Now I need to put some insulation in! I'm now running at high rpms and the noise is fatiguing.

Any suggestions on sound absorbing material?

[Guest625101138]

Originally Posted by MKP

14-15kts seems to be the sweet spot.

Now I need to put some insulation in! I'm now running at high rpms and the noise is fatiguing.

Any suggestions on sound absorbing material?

I have not actually directly purchased soundproofing materials but I have had experience with soundproof generator enclosures. The stuff I have seen I believe is dense glass fibre with a perforated aluminium sheet to hold it in place. I have attached a photo of what I mean but I could not find a reference for this. It is very effective and the aluminium is heavy enough that it does not dent if it gets handled.

Here are a few sites I found while looking for the aluminium faced stuff:
http://www.soundprooffoam.com/raf-melamine-foam.html

http://www.marinefoam.com/marine-gra...ink_type=image

http://www.noisestopsystems.co.uk/So...g_material.asp

As far as foam goes I would be very careful about the fire rating. I would want to understand more about the UL or FM classifications regarding flammability. Some foam has fire retardants and will not burn with a naked flame but they will produce fume under high temperature and this will explode if it gets to the right conditions.

I am sure there are others here who have first hand experience with sound proofing.

Rick W

[petergt]

Hi MKP,
Without doing detailed noise measurement and analysing the frequency of the noise in the boat, it is difficult to be 100% certain what the source of the noise is.
My guess would be most likely due to the 5% blade clearance causing blade pass vibration and consequent noise, which is a common problem with high speed boats and with low prop clearance. This can be difficult to get rid of (particulary given the high amount of energy forcing the vibration and noise). If you imagine the high amount force generated by a single propeller blade passing the bottom of the hull



Options to fix include
1) Increase the blade clearance (assume this is not a good option for you)
2) Try to see if the flow into and above the prop can be improved,as this can increase the problem. It is not so easy to see from the photo, but the struts look to be fairly blunt, that could cause more turbulent flow into the propellers and hence increased noise and vibration.
Poor tunnel entry (or separation in the tunnel) can also affect this, but your tunnels appear to be fairly long
3) I am assuming the struts are fairly rigidly mounted, but this should be checked

It is best to fix the problem before it gets into the hull as once in the hull it can travel as structure borne vibration to other parts of the boat, and pop out as more noise.

Putting your hand on the hull near the prop can sometime provide an indication as to where major vibration areas are occuring, but this is pretty rough. To provide a proper analysis however a noise consultant can bring a noise and vibration meter to tell you what the source of the problem is suggest what should be done to improve this (preferably one that knows what a boat is - Local shipyards may know people who can do this). Although there are no guarantees here, this is a better approach as sometimes you can tackle areas that are not the source of the problem and waste your money.
The other possibility is to find local companies experienced in supplying noise materials for boats and get their advice (cheaper, but may not adress the source of the problem).
It can get complicated and they can give you an idea of the materials that can be used to help solve the problem. Get good advice from experienced people as noise fixes tend to have a high failure rate for various reasons. Ask for references and speak to the boats owners to see if their past noise fixes worked.

Once into the hull there are 3 main types of noise materials utilised

1) Noise insulation - Like noise foam. Good and absorbing airborne noise, but may be of limited use for blade pass problems
2) Noise barriers - 5kg/sqm vinyl sheet is one type and reduce a certain amount of airborne noise travelling through them ie may be 100 dbA on one side of BHD and 80DbA on the other hence 20 dBA attenuation. The heavier barrier the better. Concrete is a great noise barrier and hence often used in buildings to stop noise. This are OK on high speed boats, but given the weight aspect they are used in limited quantities. Any hole in a noise barrier will short circuit the barrier, significantly the effect of the barrier
3) Noise dampeners - Often fixed/glued directly onto structure to reduce structure borne noise. Thickness of this varies with structural thickness of the member (often in car panel to quieten cars)

Often to fix to noise problem varying combinations of the above 3 will be used. The best solution is to minimise the amount getting into the structure as it more expensive to remove once it becomes structure borne vibration

Hope this helps

[MKP]

Wow Thanks guys! That was a lot of info. Most of my noise is engine related now. Before the modifications I had a ton of vibration and noise from the wheels, but now I currently have 18% prop clearance and the noise is minimal.

The turbo's are the single most apparent offender.

I like the aluminum and glass down deadening material however, I believe I do not have the space in the engine room to maneuver panels into place.

Does anyone have experience with the foam, lead, aluminum foil material? Is it flame-retardant?

[DBM]

I suggest you use pick-up truck bed liner spray. THis works especially well on sheet metal. Fill up any gaps and through bulkhead holes and spaces first with fire resistant expanding foam. New rubber seals on all hatches and doors really makes a difference too.