Ballast Positioning

[alandav123]

Hi all, I am experimenting with putting some ballast in the bow to offset a rather large outboard recently fitted. Is it better to place the ballast right in the middle OR should I split the two 25 kilo weights up and put one port and the other starboard?
Its quite a light boat (17ft Pilot style Dory) as it is single skin only, so she does give a bit when boarding her , so I am not sure whether placing the weight smack bang in the middle OR splitting it would help with the "giving". It would be nice if the ballast could bring the bow down a tad and also give her a bit of a steadying hand too. My gut instinct tells me it should go in the middle and the gravitational force exerted downwards will steady the boat from dipping unneccessarily when boarding. Almost like a bottom heavy kids toy that will always stands up.
As an aside, I have noticed that when I board the two Dory boats either side of me with exactly the same cathedral hulls, they give much less than mine. Mine is a veritable rock and roller. I think this can only be down to the weight they carry as they have more cabin area and equipment etc. I feel that the additional weight is making them more stable. The hulls are the same and I assume they would displace the same amount of water so I can only account for thier minimal movement due to simply more weight steadying them.

Regards and thanks

Alan in Arbroath

 

[Bajansailor]

I would be very reluctant to put any ballast at all in your boat, and especially reluctant to put any in the bow.

You will then be going around in a vicious circle, where the extra power just about copes with the extra weight......

It is probable that the two other Dorys next to yours could be heavier (and hence displace more water) - but is this an advantage? If you all have the same power, and similar hull size / form, then your boat should be faster (and hence more fuel efficient) than theirs.

50 kg is a small person - IMHO people are much better ballast, because they are easily portable (you can tell them where to sit), you can usually engage them in decent conversation, and most crew can brew up a pot of tea and bring nice sandwiches along....... your two 25 kg lumps are not going to do any of this!

And if you go out singlehanded, do not worry - your hull form should have very good stability, never mind that it might seem to be more 'tippy' than it's neighbours.

BTW, a good indicator of a vessel's stability is it's roll period - the shorter the roll period, the more stable it is. I would guess that of your dory and her neighbours, yours might possibly have the quickest roll period.

 

[William_H]

It would be far better if you can move equipment further forward ie the fuel tank and battery. Small boats suffer a lot from stern deep because the crew tend to sit well back. So even an extension on your outboard tiller with you sitting further forward might help. I clamped a piece of large sized rubber hose on my o/b tiller so throttle and steering can easily be done from further forward.

Balast mounted forward of centre(ie in the bow) will tend to encourage pitching although of course the further forward the balast the more balance moment it has so less weight can do the job. So it really is worth moving equipment forward.
I am not sure if you sail this boat or just use it under motor. Boats can be intially unstable but then get good stability from the chine area settling in the water. So your initially tippy assessment may be unfair.
Anyway to answer your question. The balast can be in one or 2 parts. Two parts sounds easier to fit. Make sure it is well secured in the bilge so it will remain in a capsize and of course make sure the structure under the balast is strong. Try various psoitions but avoid going too far forward. i hope there is sufficient air tanks to ensure unsinkable.
In a dispacement boat the balast won't make much difference to speed but may give you a more comfortable ride. But then there may be more water coming over the front. So it is all a compromise. good luck olewill

 

[boatmike]

Is your problem static or dynamic? A static trim can only be corrected by ballast but if you are worried that the bow of your boat lifts burying the stern when under power try changing the angle of your outboard leg on its mounting. The further forward you move it on its mount the more you will change the angle and get a downwards component to the driving force lifting the stern and lowering the bow. If the weight of the outboard alone sinks the stern a considerable amount it may be too big for the boat and dangerously overpowered. What O/B is it?

 

[alandav123]

Hi Guys, Its a low hours 90 4 stroke mariner out board so it is large for that size of boat. I have had widely differing opinions as to whether it is or is not too big, frankly the debate has worn me out. Today I have the guy who sold and fitted the outboard coming down to sea trial the boat and decide one way or another if the O/B is suitable. I really hope everythings is fine.
The ballast I used is two hessian bags with 25kilos of small round beach stones and they have gone in the forward hatch with the anchor etc. They are pushed as far starboard and port as possible to evenly distribute th e weight. The boat does sit higher in the stern and looks better all round.
I am really hoping to rersolve this issue today once and for all. I have been trying to get it sorted one way or another since November, but put it off due to a new baby arriving.
I suppose I was swayed to some degree when I bought the Mariner. The other engines for sale had unknown hours and no service history whereas this one had full documentatiom , only used under 160 hours and the seller had sold it new and always looked after it. So I was buying its history as mush as its performance, it was also warrantied.
The price was also under half what a new one would cost so I thought it was a great deal. But the other boaters here were headshaking from the get go, saying its big etc..I posted here and many said they had used these pilot style boats with 90hp and even 120hp O/B. So it is very hard to come to a decision ..especially when you dont have a long sea going background.
So its fingers crossed for todays sea trial.

I suppose if its deemed too big, I could always sell the Mariner for decent money as its an as new 90 4 stroke immaculate, serviced, warrantied and under 180 hours, so not all is lost.

Rergards Alan in Arbroath

 

[Bajansailor]

IMHO 90 hp is not too big - there are quite a few 17' Boston Whalers here, and 90 hp is about the minimum size engine that they use (mainly for water-skiing).

Just because you have that power, doesn't mean that you need to operate it flat out all the time.......

All else being equal, all boats are always happier when operated with a bit of trim by the stern - improves their directional stability, especially if say you are running down a wave, which I am sure you will be occasionally. Same as how raggies keep weight off the foredeck when sailing, it is always a good idea to keep weight out of the bow on 'small' mobos as well.

Keep your ballast weight allocation for crew and stores - or if you really want a bit of extra weight, carry it in the form of safety equipment, eg a heavier anchor with a longer length of chain.

 

[boatmike]

It sounds rather large frankly, but I am not an expert on small planing hulls. I do think however that if as you say you are not very experienced you should be very careful especially in any kind of sea where the slightest wave at full power could flip the bow. Under these circumstances a little bow ballast would not go amiss in my opinion...

 

[bedouin]

I don't think the difference between one weight in the centre or two at the sides will have a noticeable difference in any way, so which ever is more convenient from the practical perspective

 

[alandav123]

Hi all, Good news at last. I had the Guy who fitted the engine down who runs a very firm locally and we went out today for a run around. He was very happy with the baot and found no problems at all.
As luck would have it I also met up with the RNLI sea safety officer who is also a very experienced power boater, and we also went out for a real thrash around too. he also said the boat was well balanced and saw no reason that the 90 should not remain on there.
So all in all, its been a great day out and I can order the RYA MB 1 & 2 course materials as I will start taking the course soon. Also I can start getting all the wee jobs done that have been getting put off until the engine suitability was decided.

Thanks again to everyon for your helpful input.
Regards Alan in Arbroath