Davits & Loads

[Gludy]

I am being told two different things about the maximum load on davits by the marine industry.

View 1 - A pair of 100kg davits can only take a maximum of 100kg load.
View 2 - A pair of 100kg davits can take about 160kg load.

The davits in question state on them "This davit is rated to 100 kg maximum load" It does not say These davits but this davit.

Given that is the case and allowing, with abelly sling, for an uneven load on the davitts, I would think that a load of 160kg could be safely supported by the davitts. So I think View 2 is correct.

Providing the tender is supported by a belly strap and is held tight with chocks to stop it bouncing and thereby imposining large instantaneous loads, surely they should be able to support this load.

Has anyone any experience of this? Any comments?

<P ID="edit"><FONT SIZE=-1>Edited by kimhollamby on Fri Oct 5 15:45:11 2001 (server time).</FONT></P>

 

[hlb]

Probabley right. Thing is with an outboard, one davit is takeing nearly all the weight, 160kg might be 140 on one davit and 20kg on the other. Also like you point out, bouncing over the waves could double the force. And then maybe the davits wont bust but the back of the boat might!!
Putting a belly strap under the dinghy wont alter the fact that all the weight is at one end. Anyway the idea of a tender is one you can drag up a beach so davits are normaly tons strong enough.
With 160 kg on the back your boats going to loose about 10 knots and stand on its arse all the time.
Or are you thinking ahead and carrying your own personal life boat around with you??

Haydn

 

[Gludy]

I am looking at purchasing a 3.4 metre Rib with a 25hp outboard so that this can be used as more than a tender.This is to go on the back of a Princess 385 and weighs 160 kg. What effect do think such a load canterlevered out of the back would have on the performance of the Princess (twin 306hp Volvo engines).?

I know such a load can be taken as such a rib is being taken off the boat prior to my purchase this Friday. However, I did notice that top speed was about 23 knots. The boat has a 6 hp generator in the lazerate and a desalniation unit there as well. It also has rope cutters and some growth on the bottom (last defoulded in March 2001).
So I am about 5 knots down on top speed. I guessed that 1 knot of this was due to the rope cutters, 2 or 3 knots to a dirty bottom and some to the extra weight of the tender.

As I need to purchase a tender, I would welcome advice on this subject - I do not wish to interefere with the boats performance too much.
Kind regards,
Paul

 

[hlb]

Thats not a tender its a bloody battle ship!!
Now a tender is some thing with a 4 or 5 hp motor and designed just big enough to take you to the beach or more importantly. To the pub and back.
Thats why there called davits.
Yer boats not suposed to be a tow truck with a bloody great crane on the back!!
Strongly suggest you put the princess in the dinghy and steer from the fly bridge with two bits of string down to the outboard!!
Much safer cos the back of yer boat might fall off at any minute.

Haydn

 

[Gludy]

Re: Davits & Loads - What effects boat speed?

you may be right but I take delivery of the boat this Friday without anything on the davits.... what do you think the effect on performance on that battleship on the back has been?

 

[Guest]

Hi Gludy, an Avon 345 seems a bit too heavy for a 385 my friend has one on the back of his Ocean 42, he is Mr Whitall davits , give him a ring for advice on 01438 746448 and by the way is your new boat called seafresh of Romsey? I have a Princess 388 and my 310 rib and outboard are plenty big enough to loose speed regards Paul

 

[Gludy]

Thanks for the advice, no my boat is not Seafresh.
How much speed do you reckon you lose with the 310 on the back (with what outboard?).
Kind regards

 

[hlb]

But, But, But You just cant do that!!
As the man said.
It's fer dinghy's
Not fer Battle ships!!
Speed lost is one thing but but!!
Oh hell whats the matter Ok bolt a 100 horse motor on the back of a concrete dinghy if you want and go like stink,
I'm shure everithing will be quite ok and will have little effect on speed stability or anything else.


Haydn

 

[Gludy]

I am not doing that - just explaining that this was what I was advised to do and seeking advice. I do not have anything on the davits at present. I am trying to decide what is best - hence my previous messages.
In a nutshell then, assuming an small rib - what size rib and engine with what total weight would be reasonable for the boat?

 

[hlb]

Well best is nothing, But you cant do that so some thing and the smaller the better. I have an avon rover and 4hp which is a bit on the small side but then again we can just about pull it up the beach and dont struggle too much pushing it down through the mud at low water..
So I'd say Avon 2.5 rib with maybe 5- 6 hp on the back would be about right, course we all have oppinions and dependes on circomstance.
Yuv got a desent boat but its not the Ark Royal give it some respect.

Haydn

 

[Guest]

Re: too much for a 385

I would say that this is too much to hang off the back of a 385, Gludy. Generally on that size maximum danglable mseems to be 2.8 inflatable, 10hp, and even then it causes problems with breaking straps and so on. Mind you. if the other gut had the same, what the heck. The smooothness of hull is most important, ropecutters might make a knot, perhaps two, tho for top speed you have to thrash and thrash and wait and wait whilst really leaning on the controls, or send wife down to help wrench the lever over. Big weight on the back might lift out for less wetted area, so it may not make much difference in top speed.

 

[kimhollamby]

Re: Davits & Loads - What effects boat speed?

Hard to say...you've identified a number of issues that might all have caused the boat to go slower than its optimum - fouling, rope cutters (which can make anywhere between 0-3 knots difference according to tests depending on boat, props, cutter blade location and so on), the gen set in the back and also the fact that the boat is a few years old now and carrying a little middle-age weight.

Thing to concentrate on is what the engines are pulling. I haven't got any specs to hand here in my office but I think from memory that the 306hp 61s are rated at 2800rpm maximum (but plse check). You really shouldn't be seeing anything much further than 50rpm either side wide open throttle. If you are well under (a likely scenario if the boat is overloaded for the original prop size and /or if it is running in particularly warm air ands sea temperatures) and possibly seeing a bit of black smoke then you need to think about shedding weight or repitching the props (but check that fuel filters and air filters are clean first and engines are generally healthy before taking drastic action). If you sea trial without the original big dinghy on the back and with low water and fuel aboard and the engines spool up over the rated maximum then it's a fair bet that the previous owner had props fitted to cope with the load. Extracting the most efficient performance from the engines is the key to best performance.

Regarding weight on the davits, I never got around to doing the test during my time at MBM but I always wanted to fit a load cell on a davit to measure shock loads at sea. This is the aspect that will kill a davit and/or its hull attachment point. In fairness to manufacturers I don't really know how you could rate for this in any kind of meaningful way but suffice to say most davit failures occur when you hit a big wave, not when you are dropping or retrieving the dinghy. If the boat has previously been used to carry a heavy dinghy, thoroughly inspect the transom areas where the davits are mounted and look for ripples in the skin of the davit itself, a sure sign it has been overstressed (look around marinas and you'll see a fair few examples). One answer of course to reducing loads if caught in heavy weather is simply to go slower...that may or may not suit you.

Incidentally, try entering davit in the search option for these forums (select all forums and all posts) and you'll find some previous comments aboput size of tender that might be helpful.

 

[traceyandsteve]

Gludy, we made exactly the same enquiries when we bought our new rib a couple of months back. After extensive questioning of rib suppliers, local chandlers and Princess themselves, we were told that the davits on our 385 would take a maximum 3.0m rib with a 15 horse engine, which is what we bought.

We came back from Den Helder at the end of August, albeit in a rough sea (force 6 with a 2m swell) and we only managed about 15 knots all the way back. Rib was bouncing about all over the place despite securest possible ties, and sure enough, we have some serious stress fractures on the davits.

We're now waiting for the insurance company to tell us that we're taking too big a load on the davits.

I would definitely have thought (on our experience) that the rib you're talking about is way too heavy. Of course, I suppose it does depend on the davits though!

If you've still got our phone number, give us a ring if you want to chat about it.

Tracey

 

[Deleted User YDKXO]

Gludy, talk to any davit manufacturer and they will tell you view 1 is correct. They reccomend that the total load of dinghy and engine does not exceed the safe working load (swl) of ONE davit for reasons that have already been given ie. unbalanced weight and shock loading. I carried an Avon 3.4 with 15hp o/b on the back of a Princess 435 with 150kg davits and I still managed to break one davit and crush the bearings in the other
I agree with Byron. Why do you need such a large tender? Apart from davit loads, its a problem to drag it up a beach or carry it anywhere.
The other thing to consider is the RIB itself. They vary enormously in weight (and cost) depending on whether they have a single or double skinned hulls. I reckon you dont need a double skinned hull unless you're practising for the SAS
I've learnt my lesson on dinghies and davits. I have a Valiant 300 RIB with 4hp 4-stroke o/b (no messy oil). It's single skinned and light enough for 2 people to drag up a beach or carry and has enough space for 4 or 5 adults

 

[hlb]

Panel V Gludy Court Of Appeal. The Saga Unfolds!!

The verdict
Your not allowed 25hp cos thats only ok for S.A.S So you might get hunted down as a terrorist and shot!!

You cant have 15hp cos they break 150kg davits so it follows that 10hp will break 100kg.

But then your in Swansea with 90 miles to go in enormous seas. So knock a bit off for that.

The court sentences you to go directly to Wolworths (without passing GO!!) and purchase one of thoughs jazzy
little numbers with sharks teeth on the front.

However because of extenuating circumstances and taking into account your new boat.

You can just bloody well Put-Put about in a little rubber dinghy and look as daft as the rest of us.

I rest my case.

Now Next case please!!


Haydn

 

[Gludy]

My Sentence

After reading all this advice, I accept my sentence which is:-
1. Limit size to 3m.
2. Go for a rib with a single skin bottom.
3. Go for a maximum of 5hp engine - four stroke.
Does this sound sensible?

P.S. I can always get an artist I know to paint sharks teeth on the front of the rib - or maybe buy one of those things from Woolies, cut the teeth out and stick them on the rib.

As regards the 'down' on Swansea, the tide range is not the biggest in the world - only the second biggest. The channel obliges by showing its botttom to you all the time .... all this makes for interesting boating does it not?

I just hope a 5 hp engine can cope with those tides - if I find myself being swept away without even making it to shore, I have left instructions behind to sue all the contributors to this thread .... but then maybe thats the plan ... a get rid of Gludy conspiracy.... when I am swept ashore in Ireland in my 3m 5hp/put put , I will return.

Tommorrow the 3.4 m rib with central support, wheel and 25 hp engine full of fuel comes off the boat as I take posession - it seems to me it will not be a moment too early!

 

[Guest]

Re: Appeal Court

praps 10 hp for good behaviour, and nasty big swansea tides, the worst in europe. And those Valiants of mikefs are ver light indeed. So you can have that too.

 

[Gludy]

Re: Appeal Court

Thanks so much for the appeal verdict - you must be defence lawyer and judge as I never actually appealed - just took my sentence on the chin!!

But nope .... (looking down and shuffling feet) .. I am going to stick to 5hp ... so there!!!!!!!

Paul

 

[jfm]

character witness re Valiant

IMHO get a Valiant not an Avon. They're very light, good quality (I've had a couple of them recently) and much cheaper than Avon/Zodiac which is important because all RIBS get dirty from exhast fumes and impossible to clean so you need to chuck em out after a few years anyway. Suggest Barnet marine for Valiants, eg 2.3m about £750 I believe. The Mercury 6hp 4 stroke is a brilliant engine, only a bit heavier than the 4-5-6hp 2 stroke but miles better (I have one of each, both under 2 yrs old, so I can comment in side-by-side test mode). Also the cheapest source of Mercury (and Yam and Mariner and some other outboards) is Barnet. Dunno whether you care, but Valiant is (I think) now owned by Brunswick (owner of SeaRay, Binliner, Mercury, Sealine, etc) and Avon was recently bought from Cinven (the people who own ipc and MBY and currently are selling it to AOL) by Zodiac. Yes I know, that's totally useless info.

JFM

 

[Deleted User YDKXO]

Re: character witness re Valiant

Agreed. I've had a couple of Valiant 300's (with Yam 4hp 4-stroke) and they're cheap enough not to worry if the dog vomits in them. Sure they're single skinned and the tubes are PVC not hypalon but the quality's good enough for a tender. Got a good deal from Ron Hale in Portsmouth last time. To add to the pile of useless info about Valiant they're slung together in Portugal.