Bulbous bow

[25931]

I've been told by a friend that several years ago he saw a Snowgoose fitted with a "bulge"
similar to those on tankers etc.(but obviously a bit smaller).
The one disadvantage that we find with Equinoxe is the slamming when heading into a certain type of wave & the thought occurs that a bulbous bow could be a way of reducing it.
Advice would be much appreciated.
Jim

 

[earlybird]

Search this forum for "bow bulb"http://www.ybw.com/forums/showthread.php?t=200651&highlight=bulbous+bow

 

[DownWest]

In the links, there is mention of an Elvstom 34ft . I remember an article when it was new, around 1980ish. IIRR it was difficult to make comparisons as there was no other boat of the same hull but bulbless. They liked the boat and wanted to think it helped. But it was impressions only. Not seen it tried since on a production yacht, might be a clue there. Plus a bit tricky to mould.
A

 

[vyv_cox]

Here's the only Elvstrom 34 I ever saw. .

Not sure how it could reduce slamming, presumably the intention was to improve water flow and hence speed. Presumably it will also pick up far more floating debris than a normal bow.

 

[jetwake]

Well the theory is that they reduce slamming by increasing the buoyancy, so the thing doesnt drop as hard. Whether a bulb this small has the volume to work that magic though (the old surface are:volume ratio thing)...

Bulbous bows seem to work by causing the hull formed bow wave to be cancelled by the bulb formed bow wave, but the notes I have seen suggest this really only happens, or matters, at a speed of 1.34*sqr rt waterline length. For a forty foot line this is over 8 kts and for a 25 ft line its nearly 7 knots. It seems that for most of the time a bulb will have no effect on the performance of a yacht. I suppose a racing monohull might benefit, but I wouldnt bet my wages on it.

Mind you, if they did work the 15% or so reduction in propulsive energy required would be worth having.

David

 

[jimp45]

Solaris Sunrise Sport cats., similar to a Snowgoose, have a bulb with 2 small wings either side. see: http://www.boatshed.com/solaris_sunrise_sport-boat-60211.html

 

[Bajansailor]

I think that the formula of 1.34 x Square root of waterline length for maximum displacement speed really only applies to 'conventional' hulls with a 'conventional' length / beam ratio of around 3 - cat hulls are much skinnier than this generally, hence their max theoretical displacement speeds should be correspondingly greater.

I remember seeing bulbous bows on a Prout cat (I think it was a 37) at Earls Court years ago - they were rather simple 'cylinders with nose cones' (that is probably the best way to describe them) and probably added a bit of extra buoyancy, and perhaps positively affected the wave pattern as well re resistance.

A fast power cat here had wave piercer type bows added to it some time ago - but these were to improve the entry (it was very bluff before) to make it easier for the vessel to get up on the plane.

 

[michael_w]

Years ago I sailed on a Crowther triamaran that had bulbous bows on the amas. No one seemed know if the tits made any difference apart form making people laugh.

Odd boat that one, even had a converted Reliant Robin engine as an auxillary.

 

[oldsaltoz]

In the links, there is mention of an Elvstom 34ft . I remember an article when it was new, around 1980ish. IIRR it was difficult to make comparisons as there was no other boat of the same hull but bulbless. They liked the boat and wanted to think it helped. But it was impressions only. Not seen it tried since on a production yacht, might be a clue there. Plus a bit tricky to mould.
A

No mould required, just stick some layers of closed cell foam together and sand to the required shape, then glass over and apply tinted flow-coat.
Job done.

Avagoodweekend......

 

[rivonia]

Boulbus bows

Hello, Some years ago we had a Dean Cat, it was built with the bulb on the bows from the factory. We now have a Solaris sunrise without the bulbs.
The bulbs certainly REDUCE wave slam and give a little extra speed. We found this uot as one of the bulbs was very badly damaged so we removed the pair of them. Thats when we noticed the difference. By the way they were hollow fiberglass and screwed on then glassed over again. Hope this helps

Peter

 

[grafozz]

bulbous bows

I have looked into this type of bow on multihull forums and the general consensus is that they only work when the bows are submersed and steady , ie. on a ship at a constant speed.They will dampen the pitching a little but the drag of rising and falling is also drag on the forward motion . I chose ,instead to lengthen my bows which had the same effect on pitching reduction and added to waterline length which increased hull speed . have you thought about re- shaping the Prout bows to vertical ? , this will not only increase waterline length and hull speed but give more resistance to pitching AND digging in the bows when surfing, a great safety factor . I saw a Prout escale with this bow extension in Spain and it looked "right " .These bows were tried in the early days of cat development and subsequently dropped as they did not produce the desired increase in speed . No production cats use this type of bow currently .

 

[fergie_mac66]

http://www.bobcatandcatalac.btinternet.co.uk/Idea.htm

here is a photo of the modified bows of a 9 catalac no details of how it works tho

 

[Bajansailor]

Ref re-shaping the bows to vertical - here is an interesting article about doing just that to a Freedom 39 - they achieved a longer, finer waterline and more buoyancy in the bow (which was the main objective).
http://www.sponbergyachtdesign.com/Wobegonbow.htm
And I think she looks much better now as well.

 

[boatmike]

I've been told by a friend that several years ago he saw a Snowgoose fitted with a "bulge"
similar to those on tankers etc.(but obviously a bit smaller).
The one disadvantage that we find with Equinoxe is the slamming when heading into a certain type of wave & the thought occurs that a bulbous bow could be a way of reducing it.
Advice would be much appreciated.
Jim

Jim, I can speak from direct experience here.
The bow bulbs were indeed an attempt to increase WL length and increase speed. They didn't result in ANY increase and in fact created turbulence due to the rise and fall of the bows. Its this rise and fall that causes slamming under the bridgedeck and will not be improved by fitting bow bulbs at all as they add almost nothing to the reserve bouyancy. When a big tanker is fitted with them it results in marginally better fuel consumption when in reasonably steady seas but in rough seas is no real advantage. Small fishing boats often have them to compensate for the extreme beam/ length ratio. If they were an advantage on our length/beam ratio we would all have them. Unfortunately they don't work!

 

[haydude]

Well the theory is that they reduce slamming by increasing the buoyancy

What? What theory?

The bulb is a feature used on large ships to reduce the bow wave (not slamming at all) thus increasing speed and/or reducing power consumption.

I was told it is ineffective on sail boats because they do not go fast enough to get the advantage and because they heel.

 

[25931]

My thanks to everyone.
I shall spend some time digesting all this info especially tomorrow because the sailing won't be very demanding as the forecast is for 2-3.
Jim

 

[fergie_mac66]

Dont think i would want one i could see alsorts of tangles with anchor rodes hooked over and round the keel

 

[rivonia]

What? What theory?

The bulb is a feature used on large ships to reduce the bow wave (not slamming at all) thus increasing speed and/or reducing power consumption.

I was told it is ineffective on sail boats because they do not go fast enough to get the advantage and because they heel.

NO THEY DON't my cat does not heel and theBulb realy worked.

Peter

 

[1955patric]

What? What theory?

The bulb is a feature used on large ships to reduce the bow wave (not slamming at all) thus increasing speed and/or reducing power consumption.

I was told it is ineffective on sail boats because they do not go fast enough to get the advantage and because they heel.

Bulbous bows work in the same way as aircraft wings. The flow of water is accelerated over the bulb as the ship moves through the water thus creating a low pressure area ahead of the vessel, improving speed and fuel consumption. Of course this only happens whilst the bulb is immersed in the water so they are less effective in rough conditions. They are not fitted to modify the bow wave per se nor to increase the bouyancy at the bow. On most commercial ships the space in the bulb is a dedicated ballast tank. I can see no reason why they should'nt work on small sailing boats although I suspect that the cost of fitting one may outweigh the small increase in performance as far as volume boat builders are concerned.

 

[boatmike]

NO THEY DON't my cat does not heel and theBulb realy worked.

Peter

Peter. Having done extensive sea trials with and without on a Prout cat and measured performance in all respects over several months I am convinced that the only effect may have been in very light airs with no sea running. Even than I am not convinced it made any difference really. As soon as there was any wave action there was a noticable negative effect in that the rise and fall of the appendage made a great deal of splashing!
You say it really worked for you. Can you tell us how fast the boat went with and without a bow bulb or was it fitted from the time you owned the boat?