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View Full Version : Consequences of putting overpowered motor on tinnie



NormC
03-09-2008, 09:43 PM
I'm lookinig to upgrade my tinnie which is used as a rooftopper.
Looking to get the most boat I can for the size I can carry, but there is of course a limit to size and weight I can manage.

For the boat, two of the shortlisted options are the Quintrex Explorer 3.85 and the Blue Fin 3.8. Both boats have a max motor power of 25HP. The Explorer is 40mm wider than the Blue Fin, but 60 mm less depth. I probably favour the Blue Fin. Although I lose a little width, the slightly deeper V should give a bit better ride in chop - in return for a slight loss of stability at rest, but it seems a reasonable compromise. But I've never been in either boat, and I suspect both would be good.

Now for the motor. In a number of brands (Honda and Mercury 4 Stroke and Evinrude E-Tec for example), the 25HP and 30 HP weigh the same. So I figure I might as well get a 30 HP. Same weight, and I don't need to work the motor as hard to achieve the same speed. And if I need the extra power, it is there.

I've seen a lot of boats on the water with motors above the hull manufacturers 'limit', but that doesn't make it right. What are the consequences? How serious are the risks?

1. Posssibly void the hull manufacturers warranty?

Others?????

Norm C

Alchemy
03-09-2008, 09:56 PM
Norm,

Why not ask the dealer to get the factory to build you one with an up-spec'd transom? That way you remove all your concerns.

Regards,
Dave.

CreelReaper
03-09-2008, 10:33 PM
Hey Norm,
Yep, I'm with Dave on this one, It isn't necessarily the weight of the motor that is a concern but the extra power which relates to extra forces being exerted on the transom.
This is a list as I see it against overpowering the hull.

1/ Loss of insurance cover or at the worst extremely expensive insurance premiums.
2/ Potential to be charged/fined if you were involved in an accident and injury to someone occurred. When inspected by fisheries or police it would be evident from the builders plate what the maximum hp should be.
3/ Loss of life on the actual hull due to extra forces. Extra flexing in the alloy could end up causing you no end of hassles.
4/ Potential for the hull to perform poorly due to the extra power being forced on it.......the list goes on.
Mate, I just don't think it is worth the risk.

Shane
ps....go the Blue finn, they build a great tinnie.

sparkyice
04-09-2008, 01:26 AM
i agree- it's not the weight, but the power.
when i bought my little merc 15hp 4 stroke, i asked about it being under powered for my boat, and was told by the dealer that the manufacturers test them stationary, fastened to a piling.
you could put any old motor on the queen mary, it wouldn't hurt the motor, you just wouldn't go too far too fast.

too much power would affect the handling of your boat to a certain extent.
a few years back a father, two sons and a cousin drowned when their boat went over.
there was the four of them, their gear, a 50hp outboard, in a 14' tinny with no PFDs
they are the only ones who know what happened for sure, but it was still a recipe for disaster

i would stay with the manufacturers hull plate specifications.

davez104
04-09-2008, 06:38 AM
I'll probably cop a bit of flak for this, but I'm not worried. The boat will probably handle a fair amount of extra power, no worries at all, just watch the weight. I think the weight is far greater issue as far as handling/safety etc goes. Here's an example, don't cringe.

I have a 3.95 Bermuda Discovery, fairly similar to the Explorer you mentioned, low sided, lightly built barra punt. It's factory rating is 20HP. The guy who owned it before me had the transom built up higher and braced. He then fitted it with a 50HP 2st Yamaha tiller steer. Thats how it was when I bought it. It was an awesome boat, got up and went like a scalded cat, as you would imagine, handled like a dream and displayed no bad traits at all. I should have left it alone. But in my unfounded wisdom I looked into what was available in the 4st world and found a second hand 40HP Honda with forward controls, trim and tilt etc. I checked both companys websites for weights and what I found said the Honda would be a few kilos lighter, so I went ahead and did the change over with a side console fitted as well. I can tell you now that the Honda is a fair whack heavier than the Yammie was, and the boat has never handled the same since. It's still not bad, but not as much fun as it was, it drags its bum a lot more and doesn't have the punch that the 2 stroke did.

I guess all I'm saying is, put as much power as you think you need on it, but be careful with the weight, it can make all the difference. And just make sure you understand the legal ramifications if something was to go pear shaped.

Good luck with it,

Dave.

Bob H
04-09-2008, 07:39 AM
I'll probably cop a bit of flak for this, but I'm not worried. The boat will probably handle a fair amount of extra power, no worries at all, just watch the weight. I think the weight is far greater issue as far as handling/safety etc goes. Here's an example, don't cringe.

I have a 3.95 Bermuda Discovery, fairly similar to the Explorer you mentioned, low sided, lightly built barra punt. It's factory rating is 20HP. The guy who owned it before me had the transom built up higher and braced. He then fitted it with a 50HP 2st Yamaha tiller steer. Thats how it was when I bought it. It was an awesome boat, got up and went like a scalded cat, as you would imagine, handled like a dream and displayed no bad traits at all. I should have left it alone. But in my unfounded wisdom I looked into what was available in the 4st world and found a second hand 40HP Honda with forward controls, trim and tilt etc. I checked both companys websites for weights and what I found said the Honda would be a few kilos lighter, so I went ahead and did the change over with a side console fitted as well. I can tell you now that the Honda is a fair whack heavier than the Yammie was, and the boat has never handled the same since. It's still not bad, but not as much fun as it was, it drags its bum a lot more and doesn't have the punch that the 2 stroke did.

I guess all I'm saying is, put as much power as you think you need on it, but be careful with the weight, it can make all the difference. And just make sure you understand the legal ramifications if something was to go pear shaped.

Good luck with it,

Dave. what sort of goose puts a 50hp on a light punt ,or even a forty four stroke???,what is it ?a power thing or just plain stupid , why would you want that much power,suppose then they then whinge about the boat manufacturer making a crap boat ( dont touch them, they,re crap, mine fell apart) bloody stupidity,do you ignore road rules as well,safety directions etc
may as well throw your safety gear away as well ,why do you think they have motor limits (so numbnuts dont overpower them and endanger themselves or others) wake up............bob

davez104
04-09-2008, 08:04 AM
It's all good, I saw that coming. I didn't complain about the boat at all, or either of the motors, just stating real world outcomes of larger than recomended motors. If you look back to the start of the thread you'll notice that was what the thread was asking about. I didn't ask for anyones approval of what has been done with my boat, I know a lot of people will dissagree with it (and probably rightly so).

I don't think you can compare it to road rules though, more like vehicle modifications if anything. I still obey all the on water rules, speed limits, navigation, safety equipment, no drinking etc etc, the same as I obey road rules. So, yeah, maybe it's just a power trip, I do own two V8 cars as well, maybe a bit of a theme?

Anyway, I was trying to give my real world experience with the thread topic, couldn't give a rats if you approve or not.

Have a nice day, I'm off to soup up the dishwasher ;)

Dave.

Red60
04-09-2008, 08:24 AM
I have to agree with Dave, weight is a factor also especially on a tiller steer boat, and if you are over 100kg's like me :-/. Although weight is not a factor in this case Shane is right, the insurance / litigation risk is probably the biggest issue here if heaven forbid something did go wrong.

NormC
04-09-2008, 09:29 AM
Just to restate, the only issue I'm talking about is power. Not weight.
As stated, in the motors I'm looking at there is no difference in weight between the 25 HPand the 30 HP.
The weight is well within the boat's transom load spec.

So it is only the 5HP step up in power that is an issue.

Norm C

pedrodepacus
04-09-2008, 09:47 AM
hey norm are you looking to buy a new boat or second hand as the step up in power is minimal but you would probably have more luck with bluefin re rating the transom than with quinnie . ive seen plenty of boats go through where i used to work that have been rated to take larger engines than normal so it can be done quite easily on a new boat.

Mindi
04-09-2008, 09:52 AM
for my two cents worth you should strengthen the transom....weight only an issue if you are heavy like me...probably only another 10kg... so how heavy the driver is may be more influence. Too much power can wipe you out if you accidentally hit full throttle/full lock .... it may well be fun on dead flat water at WOT but you may not realise how close you are to being airborne and how easily destabilised

or maybe I am just getting old and too cautious..?

Noelm
04-09-2008, 10:12 AM
I still think one of your problems is the fact that you have to carry the Motor at times when you are going out, go for the lightest possible to HP ratio

Blackened
04-09-2008, 10:12 AM
or maybe I am just getting old and too cautious..?

G'day

Probably :P

Seriously though, approach bluefin and telwater, see if they will re-rate you boat

Dave

stevej
04-09-2008, 10:33 AM
it is going against the compliance plate attached to you boat by law

i dont know what agency polices it or who enforces it but why break the law?
just buy the next hull size up or a boat fit for the purpose you require.

just too much to go wrong and you will risk your own life and maybe others

gofishin
04-09-2008, 10:41 AM
The max power is not merely a factor of how strongly the transom is built, but how much power is allowable due to the physical size of the boat according to the regulatory standards. The ABP allows aussie builders to choose between a few standards, however at this end (small) of the scale, they pretty much all give the same result – and I would guess that 25HP is probably pretty close to the limit.

From memory, according to the standards, the max hp is a factor of: length, waterline beam, type of hull (ie planning/displacement) and wether it has remote steering etc. There is also what’s called a Quick Turn or Performance Test that must be performed in certain sizes/situations etc, and this is the limiting factor.

I do not believe a builder will rate max power above that according to the standards, as it could/would be taken as them accepting a much greater liability in the event of an accident. Go to the USCG site or Google ‘USCG + max power’ etc and you can check yourself. If 25HP is at the limit then you probably have no chance.

Whitto
04-09-2008, 11:06 AM
All or most tinnies are stamped with a maximun hp rating.........anything higher than is recommended may very well be illegal.....Im not sure about that though....worth investigating

Kajiki
04-09-2008, 01:42 PM
I think a few of the guys touched upon it. Regardless of whether the transom can take it you would be leaving the door open for the insurance companies to wipe you like a dirty you know what if things go pear shape on land or water.

If they have an opportunity to wipe you they will.

goddy100
04-09-2008, 03:28 PM
So how do you get a hull upgraded? I have a 4.3m v-nose heavy duty punt with an upgraded transom, pods fitted on back, plus enough decking to make it weigh a ton. the hull is rated to 35hp, but I have a 50hp yamy on the back which pushes it just fine and, imho, would be the minimum with the extra weight. But on the plate, still says 35hp. What to do??

Goddy

dreemon
04-09-2008, 05:08 PM
Hi Norm, as for consequences / risks, I put a tiller 40 hp on a 12 ft, the only reason for that is it was the only boat I had to test this engine before buying it, way to heavy for transom but only took on a little water on take off, would mono anytime you wanted, like a zodiac,

once on the plane it was good and could cruise.... but I needed to see if it would go WOT, it ran like a unicycle outa control, the risks would be that at full power it sorta broached without warning, took it out a few more times before I put it on a 4.1, prob because it was so dammed fast and a buzz, at the end of the day, no dramas, your talking 5 hp extra on your boat and a 6 hp can get a small boat up on the plane, so i think you would notice the 5 hp, I hope this helps your q's

Whitey81
04-09-2008, 05:42 PM
Norm C
You will definately tell the difference between a 25 and 30. I guess one way around it would be too find out the difference between a 25 and a 30. Maybe as simple as a carby spacer, jets. So my answer to you would be to buy a 25 and modify to 30. No one will ever no the difference!! I cant believe no one else thought of it.
Cheers
Whitey.

PS Your boat will handle the extra HP, no one engineers something without any extra meat.

indian man
04-09-2008, 05:42 PM
25HPis plenty mate plenty8-) a mate had a 25 on a 3.5 qunni over powered endid up splitting the hull boat wasent that old

Outsider1
04-09-2008, 06:43 PM
.......................Now for the motor. In a number of brands (Honda and Mercury 4 Stroke and Evinrude E-Tec for example), the 25HP and 30 HP weigh the same. So I figure I might as well get a 30 HP. Same weight, and I don't need to work the motor as hard to achieve the same speed. And if I need the extra power, it is there............................

Norm C

As mentioned by a number of posters, voiding your insurance is probably the biggest issue.

I would also suggest approaching the manufacturer to uprate the transom as being the easiest way to achieve what you are after. There is not going to be a huge difference between a 25 to a 30 but potentially it could be worthwhile. A Marine Engineer would also be able to assess uprating and certifying the hull for you.

It is an interesting issue particular with Evinrude's range of HO motors. For example the new 115HO is said to have almost identical power output to the 130hp and will be basically the same price. So someone with a boat rated to 115hp max can put on a 115HO legally and have similar performance to having the 130hp on it.

Here are a couple of videos I found showing over powered boats and how they go. Not what you are looking at, these are significantly over powered. I have also seen a couple of some guys larking around with a 12 foot tinny with a 70hp on it but can't find it on you tube at the moment!!

4BO6EbCTKwM

OMo7qvmsMu4

b1O8wfYaJCw
Cheers

Dave

finding_time
04-09-2008, 06:54 PM
25HPis plenty mate plenty8-) a mate had a 25 on a 3.5 qunni over powered endid up splitting the hull boat wasent that old

I agree 25 is plenty!!! People are really turning into rev heads on this site!;D It about having a balanced outfit i reckon!;)

Ian

indian man

A near new quinnie split you say? Really , now who would believe that!!!!!!::) ::) ::) ::) ::) ::) ::)

BrenMac
04-09-2008, 07:32 PM
Norm C
So my answer to you would be to buy a 25 and modify to 30. No one will ever no the difference!! I cant believe no one else thought of it.

Been done plenty of times, but with a new motor you will void the warranty doing this.

Cheers
Brendan

foxx510
04-09-2008, 08:25 PM
If it's the motor I'm thinking of, the Honda BF25-BF30, then there is a lot more than a carb spacer derating it. It's carb jets, ecu, cam at the very least. I'm not sure I'd bother with the 30 though, it's only 500rpm at the top, same capacity. Possibly marginal midrange gains, but I doubt I'd be bothered strengthening a transom for it.

seabug
04-09-2008, 08:34 PM
Norm C
You will definately tell the difference between a 25 and 30. I guess one way around it would be too find out the difference between a 25 and a 30. Maybe as simple as a carby spacer, jets. So my answer to you would be to buy a 25 and modify to 30
.{{{{{{{{ No one will ever no the difference!! }}}}}}}
I cant believe no one else thought of it.
Cheers
Whitey.

PS Your boat will handle the extra HP, no one engineers something without any extra meat.

The boat would be flighty at speed and hard to control if you needed to ajust direction quickly(Accident avoidance)

If you were unlucky to be in an accident the extra power would be noticed by the investigators,and even if the accident was caused by the other party you would in a lot more strife than just with your insureance company
Regards
seabug

CreelReaper
04-09-2008, 10:35 PM
Very interesting comments guys. Just to add a little more to the think tank. Yes it is an individual's decision to do what they want. But at the end of the day when something goes wrong all HELL will be there to pay. I can't say all but a lot of boat manufacturers will have test documentation of the grades of aluminium they use in their hulls. This information covers things like Tensile strength, elongation, ductility/hardness but the biggest killer would be the amount of load cycles the material will be able to handle for known maximum loads. Guys I have worked in a testing laboratory for many years and it continually opens my eyes to what can happen to a metal when it has been worked too hard (fatigued). Now I am not too sure on specs etc but the boat designers will have a 'lifespan' for a hull of so many years. This takes into account how many cycles (flexing in the transom and hull) that it can take for a maximum hp rating. Aluminium will easily take the recommended cycles for many a year but eventually stress cracks and fatigue failure will start to appear. Weld integrity will offen decrease and finally a split/metal failure will occur. This is similar to planes in that it is all worked out to the "n" degree. Now it might be all fun and laughs but when that incident does occur and someone is killed, then don't think for one minute that the relevant authorities will not be diligent in their inspections. (Use the tv show air crash investigations as an example) Inspections on motor and hull/fittings will be done and the testing will be thorough. If it is deemed that you have deliberately used an overpowered motor or have had alterations done to 'improve the hull' without an approved engineers certificate then heads will roll. May not be murder but manslaughter charges could possibly be laid to the guilty ones. Just think, dad takes son out on the water in their "souped up" little boat for a lark, lots of fun until.......transon is ripped off the boat. Boat goes down and son is lost????? I know its a negative thought but is it really worth it?????

Shane

patrol50
07-09-2008, 12:04 PM
hey norm i have thought about a similar idea re upgrading from my bumada 3.75 vee punt car topper on the nissan - just a couple of points for you to consider - if i go that way i would go for a long shaft OB - and have boat manufactured with higher strengthed transom ( probablu have to forget about the quinny then as they build standard boats ) you dont want a wave over the back if you have slow down suddenly - have you considered how you will move and lift the big motor 25/30 HP are bloody heavy to move around for one person mate - have you tried it, i have - also have a look at the aero floats for extra stability for the deeper vee blue fin to tidy up the stability at rest they do work- i have a set of 2.0 m for my bermuda - also have a look on the F & B website sea media as in one of the back issues was the calcs how to work out max HP for a boat of a certain size
best of luck see you rob

NormC
07-09-2008, 07:47 PM
hey norm i have thought about a similar idea re upgrading from my bumada 3.75 vee punt car topper on the nissan - just a couple of points for you to consider - if i go that way i would go for a long shaft OB - and have boat manufactured with higher strengthed transom ( probablu have to forget about the quinny then as they build standard boats ) you dont want a wave over the back if you have slow down suddenly - have you considered how you will move and lift the big motor 25/30 HP are bloody heavy to move around for one person mate - have you tried it, i have - also have a look at the aero floats for extra stability for the deeper vee blue fin to tidy up the stability at rest they do work- i have a set of 2.0 m for my bermuda - also have a look on the F & B website sea media as in one of the back issues was the calcs how to work out max HP for a boat of a certain size
best of luck see you rob


Rob,
Some fair comments.
Yes, I plan on going for a long shaft motor.
If I go for a 4 stroke or E-Tec 25 or 30 HP (prefer 30 HP), the motor will weigh around 70 KG. Not light. It will be carried on a fold down motor bracket on the draw bar of a caravan. After tilting the bracket up and moving the boat as close as practical, I fugure we have to lift the motor and move it about a metre, then lower onto transom.

I plan to give it a go (befor purchase if possible). If wife and I can manage it, that's how we will do it. I manage the 20 HP Yammy 2 stroke (about 50 KG I think) by myself at the moment, so we will hopefully be OK with the extra 20KG. If not, I have a design in mind for a fold up crane.

I've heard of the aero floats, but no experience with them.

From the specs, I'm figuring the 3.8 Blue Fin is not as deep a V as the 3.75 Dart as it is a touch shallower and a lot wider. So not sure we will need them if we go for that boat. The Blue Fin web site says the 3.8 is only available in short shaft. I've asked a Blue Fin dealer if it can be made in long shaft (and upgraded to 30 HP), but have had no response.

Another option is the Stacer 3.99 Proline, which is even wider, comes with long shaft transom as an option and has a 30 HP max.

NOrm C

patrol50
08-09-2008, 03:20 PM
norm that stacer proline looks interesting mate thanks for the tip on that but sadly at 111kgs is a bit heavy to car top on my nissan i think - mate an ob of 70 kgs is a hell of a lift even for 1 metre - i can understand your desire for a 4 stroke i have a 50 hp 4 stroke on my quin 470 (its only a merc ( newish) but still better than a 2 stroke) - but mate on a move about tinney the lighter the ob is good in my opinion so if i do go that way i will go with the lightest 25 / 30 hp i can find - the yamaha premix 25- 2s 48 kgs or the merc mlh lite 30 - 2 s 48 kgs - prob bit heavier in LS just my thoughts mate - i find the aero floats do give me a bit of extra comfort zone in a nasty chop - still get bloody wet though
my best regards rob

goddy100
08-09-2008, 04:01 PM
can't be that bad, in the latest NAFA they do a review on a stacer barra pro 519, rated to 100hp, then show piccies with a 115hp on the back. It's probably under the max motor weight though, so which one is the most critical, weight or power?

Goddy

NormC
08-09-2008, 08:16 PM
norm that stacer proline looks interesting mate thanks for the tip on that but sadly at 111kgs is a bit heavy to car top on my nissan i think - mate an ob of 70 kgs is a hell of a lift even for 1 metre - i can understand your desire for a 4 stroke i have a 50 hp 4 stroke on my quin 470 (its only a merc ( newish) but still better than a 2 stroke) - but mate on a move about tinney the lighter the ob is good in my opinion so if i do go that way i will go with the lightest 25 / 30 hp i can find - the yamaha premix 25- 2s 48 kgs or the merc mlh lite 30 - 2 s 48 kgs - prob bit heavier in LS just my thoughts mate - i find the aero floats do give me a bit of extra comfort zone in a nasty chop - still get bloody wet though
my best regards rob

Rob, I've got the 20HP 2 stroke Yammy - same weight as the 25Hp you refer to. I can manage it by myself, but much easier with someone else.

Yes 70KG is a lot of weight, but I work on the basis that these days, when we stop to fish and set up the boat, it is normally for at least a week, normally longer and sometimes a couple of months. We have a fold up trailer, so the boat stays set up while camped.

2 mins of heavy lifting at the start and finish of a 2 week or more stop, in return for benefits like:
50% fuel reduction (as much as $1,000 per year with our current usage pattern)
much quieter operation, particularly if trolling
way better for the environment.

If under test, we can manage the 70KG of the 4 stroke or E-Tec, that's the way we will go. If not, the next option is to set up a folding crane to help with the lifting. If none of that works, it lbe the Yammy 25 HP 2 stroke. But I'd really like a 4 stroke or e- Tec.

The main difference is that I have a new 70 Series ute with GVM upgrade to 3,700KG and 200 KG roof load capacity. Carrying the load on the vehicle / caravan is not the issue for me. Just the manual lift to transom and back.

cyclone8888
08-09-2008, 09:41 PM
The main difference is that I have a new 70 Series ute with GVM upgrade to 3,700KG and 200 KG roof load capacity. Carrying the load on the vehicle / caravan is not the issue for me. Just the manual lift to transom and back.

Hey Norm,

Mate sorry for the Hijcack, just wondering how u went about getting your GVm upgraded?? How much and are u in QLD??

Just about to shell out for a new 70 series and would be very interested in how u did it.

Cheers Mate.

PS I have a 4.1m Tinny with a 50hp yammie 2st, the boat doesn't have a plate on it that i can find... The current models have a max rating of 40hp, whilst this one does have a fair bit of extra alloy in it flat out it is a handful - generally sit on about half throttle and 20 knots, mind you that extra power is great for getting it out of the water loaded up... IMO I probably wouldn't go over the suggested maximum next time around, but really only for insurance/legal reasons.