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Discussion Starter · #1 ·
Our club has a converted ex oil rig lifeboat, which is badly underused (totally unused :nonono:) mainly because it is painfully slow compared to the other club boat (a 34ft Blyth Cat with 2 x 350hp turbo diesels)! Given that the ex lifeboat is never going to break into a gallop, I think it should be quicker than it is, & it should be more answerable to the helm, which leads me to believe that it is sitting too high in the water. Members travelling behind us the other week said that there was a lot of white water at the stern of the boat, & it was difficult to keep it on a set course, it was getting "blown sideways" by the wind, & the current was taking us off course even when the throttle was full ahead, also when we were trying to lift the anchor, it would barely go ahead against the tide to take up the slack, as if the prop wasn't "biting" into the water, but just thrashing at the surface?
I don't accept that "It's a lifeboat & was only intended to go in a straight line until it is rescued" videos on YouTube show them being dropped in their SOLAS test, then manouvering quite easily & at a reasonable speed back to the launch area, to be dropped again. The rudder on it is a tube, which acts as nozzle to direct the water, & at present steering is "vague to non existent", all these suggest to me that the boat is suffering from "short shaft engine on a long shaft transom syndrome" which I have experienced with outboards.
This boat came with a top cover (which rung the overload bell on the crane lifting it off) & apparently there were also 2 very large metal cylinders in the bilge, containing what I assume was C02? So do any of you displacement hull men have a definitive answer as to how much water needs to be over a prop to prevent cavitation? Or is it merely a "suck it & see" process of adding more weight until you think it is right? :g:
 

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load her up with club members to weight her down , if it works calculate the weight of the people and put the same amount of ballast into the bilge
 

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The photo of her in her orginal condition says she takes 50 people - that's a lot of weight! - the prop looks very high in regard to the hull - probably designed not to to foul anything during a launch rather than for optimum performance at sea. I suspect when loaded with the weight she was designed to carry, the prop would have been quite a bit lower in the water - but the gunwhales would no doubt then also be very low in the water - with her original fully enclosed (and presumably water tight) superstructure this probably wasn't an issue - it looks as though you might end up with a sitation where once the prop is performing as it should the gunwhales as she is now would be rather too close to the water. Do you have any photos of one on an exercise to confirm where the water line should be?

What happended to the orginal superstructure? - could the bottom 18" of it be reinstated to provide greater freeboard?

240z's suggestion as a quick experiment sounds like it could tell you more - just make sure the other boat is standing by and all occupants are wearing good lifejackets!
 

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Discussion Starter · #4 ·
The photo of her in her orginal condition says she takes 50 people - that's a lot of weight! - the prop looks very high in regard to the hull - probably designed not to to foul anything during a launch rather than for optimum performance at sea. I suspect when loaded with the weight she was designed to carry, the prop would have been quite a bit lower in the water - but the gunwhales would no doubt then also be very low in the water - with her original fully enclosed (and presumably water tight) superstructure this probably wasn't an issue - it looks as though you might end up with a sitation where once the prop is performing as it should the gunwhales as she is now would be rather too close to the water. Do you have any photos of one on an exercise to confirm where the water line should be?

What happended to the orginal superstructure? - could the bottom 18" of it be reinstated to provide greater freeboard?

240z's suggestion as a quick experiment sounds like it could tell you more - just make sure the other boat is standing by and all occupants are wearing good lifejackets!
Plus there have been mods made to the gunwhales to make it more "user friendly", I take in what you say about the weight of 50 people making the prop work efficiently, but also that the gunwhales would be too near the water for comfort :nono: So it would appear that the only answer is to progressively add more weight to take her down in the water, until a compromise between safety & stability/control has been achieved. I think this is something that has to be decided by the club as a whole, rather than a decision taken ad hoc. Your conclusion that the waterline was irrelevant in its original form was one I had not considered, but is very well considered now, & I will go back through YouTube & try to establish the depth at which they were sat during the SOLAS test. I doubt they were loaded with 50 people for a 30ft drop, so it is just the weight of the top, & the CO2 bottles, that really needs to be added to what ballast is already in it, to bring it to a sensible compromise. Thanks both for your contributions, you can never get enough good advice, & never have too many observations & sensible suggestions, they will be added to the discussion. :clap:
 

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I doubt they were loaded with 50 people for a 30ft drop
I wouldn't be surprised if there was equivalent weight on board for the tests though - they wouldn't be approved for 50 people without testing to ensure they were safe with that number on board.

Thinking about your club situation a bit more, the way people sue people left right and centre these days if things go pear shaped, I would also consider how committee members/club officers stand in regard to personal liability if the club sanctions a home-modified boat for club use - such issues can be far from funny these days.
 

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Ha ha this brings back memories,

I did my PSCRB (Proficiency in Survival Craft and Rescue Boats) ticket 2 years ago, part of course was taking the boats next to the davits before lifting them out, with around 7 people in the craft everyone was instructed to move aft to reduce cavitation when getting her into position.... still needed to give her a good blast before she'd react!

No idea how much weight was in the co2 bottles or topside structure is but you might need more then the combined weight to counter the cavitation, suppose as you said just comes down to required ballast verses freeboard.

Lewis.
 

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Discussion Starter · #7 ·
I wouldn't be surprised if there was equivalent weight on board for the tests though - they wouldn't be approved for 50 people without testing to ensure they were safe with that number on board.

Thinking about your club situation a bit more, the way people sue people left right and centre these days if things go pear shaped, I would also consider how committee members/club officers stand in regard to personal liability if the club sanctions a home-modified boat for club use - such issues can be far from funny these days.
A full marine survey, including heel, stability, & float tests, were carried out by a highly qualified marine engineering surveyor, & it has been approved & insured for 6 persons. All i's have been dotted, & all t's crossed in terms of safety & insurance, & lifesaving equipment. Its just this lack of guts, & poor answering to the helm that is troubling us.:nonono:
 

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Discussion Starter · #9 ·
If his services weren't too expensive, I wonder if he would be the best person to consult?
To look at it after its been done, but paying a "shall we say" fee, for a calculated guess is out, better to throw it open to suggestions from the floor on here first. Have already had observations & ideas that make the problem more understandable, & therefore some possible solutions are already on the table. It might be an idea to run the best ones past him once they have been considered, but no open cheques Clinker, you people in Tenby must have too much money!:icecream:
 

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Discussion Starter · #11 ·
Nearly all ships boats are designed to take upwards of 60 people, so if you only have 10 bodies on it then yes you need to ballast her out.
New 370-person mega lifeboat and davit system developed
For 6 (including skipper & 1 crew) so we are going to have to strip a few church roofs of lead to get her down enough for the cavitation to stop. Can we add more to the stern than to the bow to take that area down more, or should we consider taking the whole hull down equally? The attached photo shows the boat sat in the compound the black boot top is visible for about 4" above the waterline all around the hull, which indicates just how high she is sat! We need a cathedral not enough lead on a church! :nono:
 

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If you do nothing more then a tonne or more of ballast from just forward of midships to just aft of the engine with marginally more at the aft end should reach the desired result.
It may need a bit more than a tonne. Again focus the weight just aft of midships.

She would have been tested up to and beyond the weight she was designed to carry when in service and light she'll be a pig. A bit like driving a pudding bowl using an egg whisk for propulsion and steering with a stewed tea bag.

You may need to gave her recoded post adding ballast as the ballast you will need is way more than the weight she us now coded to carry.
 

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Discussion Starter · #14 ·
If you do nothing more then a tonne or more of ballast from just forward of midships to just aft of the engine with marginally more at the aft end should reach the desired result.
It may need a bit more than a tonne. Again focus the weight just aft of midships.

She would have been tested up to and beyond the weight she was designed to carry when in service and light she'll be a pig. A bit like driving a pudding bowl using an egg whisk for propulsion and steering with a stewed tea bag.

You may need to gave her recoded post adding ballast as the ballast you will need is way more than the weight she us now coded to carry.
Of how it drives & handles at present are spot on, the steering wheel resembles a mangle, whereby you turn it hard to port & it may or may not respond (depending on wind or current). Going forward against the tide & wind is like using an ashtray on a motorbike, IF it decided to go ahead it is painfully slow, & when the slack is taken out of the anchor warp & any weight is taken on the bow, the prop turns into a blender.
Little wonder the members are less than enthusiastic to go out on it, & little wonder why none of the skippers want to take it out, so an important club asset which can take small numbers of members fishing economically, on inshore trips, is tied up to its berth in the marina 365 days of the year. To see how it should handle watch this video on YouTube "Free Fall Lifeboat Trials in Dundee" that shows an identical boat responding to the helm & throttle as it should, so look out churches we need a ton according to Tom Bettle! :yeah:
 

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Hi Blueskip,bearing in mind that 50 people weigh over 3 tons, plus the top of the boat, plus the tanks you took out you are looking at a lot of weight.Bearing in mind where you live would it be possible to get a hold of a couple of tons of railway line at a local scrapyard? if you work on it weighing 110lbs per yard you will be able to measure your bilges and see how you can fit it.The lower she sits in the water the longer the waterline therefore the higher the hullspeed.Seeing you are not likely to have people and ropes around the boat in the water It might be worth putting a conventional rudder on it,
regards Dave.

boilerplate would be another option but i don`t know if you can still get it
 

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Discussion Starter · #19 ·
On this topic so far, & some of the figures bring you back down with a bump, 12 people weigh only 1 ton, 50 people weighing 3 tons +, the weight of the top section, plus those 2 cylinders in the bilge, we must be looking at the fully loaded boat carrying another 6 tons! Little wonder it is blowing around like Li-Lo. I don't know if the idea of a rudder will get much support, but if it doesn't perform very well after the added ballast has been added, then maybe it will be an alternative, again it is another positive idea to consider. :thumbs:
 

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another way to add ballast is to get steel pressings from a factory and flood them with resin but once they are in and the resin has set you can`t change your mind. Lead would be the best but at over £2000 a ton scrap price !!!
 
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