Genneker bowsprit loadings

[zoidberg]

I'm building a 'custom' genneker bowsprit in 82mm OD T6061 alu-alloy, being guided by Selden's datacharts, and reckon from them the max. allowable load at the bow ring would be around 2500lbs force, or 11 kiloNewtons. I aim to stay well within that.

gennaker-bowsprits

Does anyone have figures for the wall thicknesses/Moments of Inertia for a 75mm Selden alloy g'bowsprit, so I can do some comparative guesstimation?

 

[Laminar Flow]

To keep things simple, rig loads are generally calculated at 1lbs/sqft.; that equates to a F5.

The next question is whether your sprit is going to have a bobstay. If yes, then you have a different situation as the sprit is now under compression and you will need to resolve the forces to figure out the actual load. Once this is done the required moment of inertia is calculated via Euler's formula for columns under compression.

To make your life easy, there are online calculators for this. My brother, who has a masters in mechanical engineering put me onto this. Unfortunately, that particular site is in German so I'm not sure if that one will be of any help to you, though, no doubt there will be anglophone varieties that are similar.

Without a bobstay or whiskerstays the sprit is subject to bending forces and this is a different ball of wax altogether.

To calculate the moment of inertia for a hollow spar: I= Pi (D to the 4th power - d to the 4th power) divided by 64
D = outside diametre, d = inside diametre.

Euler's formula is: P = Pi squared x E x I divided by L squared
Where E is the modulus of elasticity which for aluminium is 10,000,000 psi, I is the load in pounds and L is the length of the spar in inches.
This describes a situation similar to a deckstepped mast, i.e. a pin/pin type situation; for a fixed/pin type situation (think keel stepped mast held at partners) the result is multiplied by a factor of 2.

I should be noted that the bending strength of the same tube (fixed/free) is very near a quarter of the compressive strength in a pin/pin setup.

If you have a bobstay, but no wiskers, then it is probably the safest for you to consider the fixed/free bending option only.

 

[sarabande]

What is your planned free (not supported) length between the outer end of the pole and the bow ring, pls ?


Have you worked your way through this:-

Downwind Load Model for Rigs of modern Sailing Yachts for Use in FEA (fh-kiel.de)

Does your figure of 11kN refer to the inward loading or the vertical/lateral ones ?

I have a copy of Marchaj's Sail Performance if you want some bed time reading, and can drop it in.

 

[Laminar Flow]

What is your planned free (not supported) length between the outer end of the pole and the bow ring, pls ?


Have you worked your way through this:-

Downwind Load Model for Rigs of modern Sailing Yachts for Use in FEA (fh-kiel.de)

Does your figure of 11kN refer to the inward loading or the vertical/lateral ones ?

I have a copy of Marchaj's Sail Performance if you want some bed time reading, and can drop it in.

While this is certainly an interesting treatise it is more concerned with down wind mast loads and, in particular, of highly competitive types. The common method of calulating rigging loads via RM at 30 degr on the wind is well established and are for regular every day boats quite conservative and plenty safe. Without access to the computer program I doubt it would be of particular value to the OP. Loads on a bowsprit are not even considered in this paper.

While Marchaj investigates the general forces and vectors on rigs, I have not found him all that useful for calculating actual loads or determining the required engineering response. In its stead I would suggest Eliasson & Larsson and even good old Kinney's Skeenes Elements of Yacht Design.

In the OP's case, if he is using circular tubing, the direction of the load vectors are pretty much irrelevant as long as the tube is strong enough to take the maximum expected sail load.

 

[sarabande]

Thanks LF, I take your point that that it is the "actual engineering response" that the OP is seeking.

I became diverted to the 'upstream' question of finding/resolving the forces in the x,y,z axes as a precursor to helping find the engineering answer.

FWIW, I have one not dissimilar solution on my boat, which is a bit longer than the OP's. The design and fabrication were undertaken by the previous owner, a RN engineer, and it works. The device integrates into the bow roller and forestay fitments.

Useful dims are:-

Overall length of tube, 800mm
Unsupported length (outer end to first U Clamp) , 460mm
Tube wall , 2mm
Tube outer diam, 48mm
Material, stainless steel, unknown type, but certainly high quality, and I imagine the stiffness is considerable.

I have been tempted to load the tube with rockets when buzzed by jetskis.

 

[Laminar Flow]

Thanks LF, I take your point that that it is the "actual engineering response" that the OP is seeking.

I became diverted to the 'upstream' question of finding/resolving the forces in the x,y,z axes as a precursor to helping find the engineering answer.

FWIW, I have one not dissimilar solution on my boat, which is a bit longer than the OP's. The design and fabrication were undertaken by the previous owner, a RN engineer, and it works. The device integrates into the bow roller and forestay fitments.

Useful dims are:-

Overall length of tube, 800mm
Unsupported length (outer end to first U Clamp) , 460mm
Tube wall , 2mm
Tube outer diam, 48mm
Material, stainless steel, unknown type, but certainly high quality, and I imagine the stiffness is considerable.

I have been tempted to load the tube with rockets when buzzed by jetskis.

Indeed, I was going to ask you what calibre it takes ...

The load model on your device is simple bending i. e. fixed/free and at 460mm it is also quite compact. The modulus of elasticity of steel is also 3 times that of aluminium.

One thing to consider with this particular project is that if the sprit fails and the tube were to buckle under load, you are not likely to lose the entire rig.

 

[zoidberg]

In the OP's case, if he is using circular tubing, the direction of the load vectors are pretty much irrelevant as long as the tube is strong enough to take the maximum expected sail load.

Thanks. Perhaps I can progress this with some more detail....

I have dismissed 'whiskers' and 'dolphin strikers' as simply too vulnerable. I have also hesitated to consider a 'bobstay' as the best angle I can achieve is about 28 degrees. The existing bronze bow fitting is insecurely fastened with just 4 screws through the grp skin, so no load other than down will be applied. I've looked instead to an unstayed sprit, and sought to spec and procure that. I'll consider a 'bobstay' if persuaded it would be worth the effort....

I've looked at Selden's chartlet, for alloy/G075, and should be able to stay well within the recommended bounds. My tube is 82mm OD. My AUW of ~8000lbs should have me within the limits for RM=25. Similarly, I expect an USL of 650mm and can provide a DBS of close to 1 metre so that, too, should be 'sufficient'.

There is no room to mount this sprit other than on the centreline. The existing forestay will use robust Dyneema strops passing each side of the sprit to secure to the existing robust foredeck fitting. - which lies between the cleats. That is not intended as a support arrangement for the sprit.

The aft deck fitting seems straightforward. A bow support/restraint fitting will be required, as far forward as I can securely achieve. A black Dyneema strop is visible where a support steel bowring needs to be fitted. I'll need to fabricate a modification of the Selden Bowring, shown below, due to incompatibility between their dimensions and mine. Securing that into the fabric of the boat is likely to prove challenging, as it is d*** difficult for me to get arthritic fingers/me and tools right up inside the bows.

I have already acquired a good furling spinnaker mounted on a torque rope, with quality furling gear, so I really want to make use of it some day.

 

[Laminar Flow]

Without having done any calculations, I would suspect that your 80mm OD tube is probably quite sufficient for your purpose and with an unsupported length of only 650mm. I do not know the ID of your tube or the size of sail you propose to fly.

The spar will be subject to bending forces only.

For spinnakers it is often assumed that one might fly them up to a F7, downwind only of course, though you might find that a bit exiting on a boat of your size (30'?) To calculate the load in that circumstance , subtract hull speed from wind speed (say 30kts - 6.5kts = 23.5kts) multiply SA x wind force (23.5 kts) x arm of unsupported length. This will also be of interest to determine the size of fastenings you will need for attachment.

Enjoy the process of figuring things out, I certainly do and I am always tinkering with new ideas and how to improve our boats performance (god knows, with a Watson there is plenty of opportunity for that).