Performance of Quintrex Dory 420 with 25hp 4st
Hi all,
I recently bought a used Quintrex Dory 420 with a Honda 25hp 4st on the back (standard open hull with no decks etc, just fuel, batt, two seats and safety gear). Overall, i'm pretty happy with the rig, but I was somewhat disappointed by some aspects of it's performance and just wanted to get some opinions from people with similar setups and perhaps some technical advice on improving performance.
The engine appears to be pretty sweet. I changed the plugs, oils & filters when I bought the engine. It starts easily and runs well. Top speed with two big guys (110kg+) on board and gear is about 35kph (20knts), which I am quite happy with. However, getting on to the plane can be quite difficult with two on board, especially into chop or headwind (and pretty much impossible with a third person). I normally have to feather the throttle, as if i try to open it up in one go the engine seems to bog down. It doesn't seem to me like it is an engine fault, but rather a torque issue - almost as if it is overloading the engine perhaps due to being 'over propped'. (Prop is a stock Honda 9.9x12).
Also, I've had a look at the engine mounting height today and it seems to be mounted much too low - cav plate about 50mm+ below the bottom of the hull. I know that the rule of thumb is that cav plate should be level or max 25mm below hull, and I have noticed that there is quite a bit of vertical spray coming up the transom when on the plane which I assume is a symptom of motor being too low. I can change mounting bolts to the bottom mounting holes which will raise the OB by another 25mm, however it will still be 25mm+ below hull and I can't go any higher without building up the transom height somehow.
Anyway, I realise this is a bit of a wide question, but like I said just looking for some opinions from people with same or similar rig or people with good techincal knowledge. To sum up:-
1. Would you say that this rig was underperforming based on the above?
2. Does the 'bogging down' engine whilst opening throttle sound like an engine fault, or just overloading the engine as I suspect?
3. Is cav plate 25-30mm below hull okay? If not, how can I get an extra inch on the transom height? It is the type with an angled cut out in the middle, so assume that I could just get someone to weld on a section (25x25mm square tube perhaps) onto the top of the exising transom.
4. Should I change to a lower pitch prop, considering I will normally be fishing with a bit of a load? What size should I go to - 9.9X11?
5. I was looking at fitting an SE Sport 200 hydrofoil to cav plate to try help getting on plane, however since the outboard seems to be a bit low already, I am worried the plate would sit under the water (rather than on top as it is supposed to) and just make matters worse by creating more drag?
Any help or advice appreciated.....
dodgy
Posts: 4578
Date Joined: 01/02/10
The 25 4 stroke on that hull
The 25 4 stroke on that hull is never going to give thrilling performance. I would lift the outboard up and give it a try but it's not going to massive change things.
I would give the foil a bit miss. They have a reputation for some strange and dangerous handling traits on tiller steer dinghys.
Does anyone know where the love of god goes, when the waves turn the minutes to hours?
tigerfish
Posts: 88
Date Joined: 20/08/10
Yeah, maybe i'm expecting
Yeah, maybe i'm expecting too much. Had a 25hp Tohatsu 2st before this and had gotten used to the free reving grunt (even if it sounded like a tractor).
Think you're right about the foil - i'm still learning my way around the big briney and 25hp is probably a bit weak with a following sea as it is without adding any other complications.
Thanks for the advice
strike_zone
Posts: 403
Date Joined: 09/09/10
4 strokes dont have the guts
mate you need a 30hp 2 stroke at least if you want to get up on the plane in those conditions atleast even a 40 wouldnt hurt but unfortunatly 4 strokes aint gunna do it for ya
tigerfish
Posts: 88
Date Joined: 20/08/10
Yeah, the hull is rated to
Yeah, the hull is rated to 40hp and think that would be the go esp with big guys and gear on board. Don't really have the money for a newish model 40hp 4st at the moment though and have gotten too used to the easy starting, smooth trolling, smoke free reliability of 4st engines to go with an older 2st.
grayzeee
Posts: 2283
Date Joined: 09/07/09
dunno if it's
dunno if it's underperforming
i had the same hull with a 40hp yammy 2 stroke , only got me 26knots flat out , but no probs getting there loaded up
If I spent half as long fishing , as I do reading this bloody forum , I'd be twice the fisherman I am.
Reefmonkey
Posts: 711
Date Joined: 22/09/08
420 dory
Have the same hull, with a 30hp 2stroke. think i have the trim set to the middle(3rd) hole which I've found is the best for an all round application. Had the transom raised recently because i had the same problem. the motor was only sitting about 40mm too low as i had a few raisers on the mount to raise it a bit. The old height threw out alot of water and with the 30 the boat had a top speed of around 24knots with two people on board and dive gear.
Found with my motor its got awesome bottom end,still only took about 15% throttle to get it planing easily with the motor mounted low though and 20knots at half throttle but the last 20% only makes noise and adds a knot or two.
Had the transom raised by 60mm and the old bolt holes welded over recently by the guys at marineline for only $150 (also removed and replaced the motor myself to save on the small amount of labour involved) have the hull level with the cav plate and still throws a little bit of water but got anouther 5 knots out of the motor.
I reckon go with raising the transom and you'll get a better top speed and may help you get it up easier, if you haven't already play with the trim holes and trim it in a bit more. I haven't toyed with the idea of a hydrofoil yet reason being its a very light boat and i wouldn't to confident with it in a following sea.
Dave J.
tigerfish
Posts: 88
Date Joined: 20/08/10
CHeers for that, some useful
CHeers for that, some useful stuff there. Will try the small lift first and play with the trim pin (currently in the 2nd to bottom hole, so will try middle hole). Otherwise, $150 to build up transom is a lot more reasonable than I thought so might try that.
Out of interest, do you know what prop you have on?
Reefmonkey
Posts: 711
Date Joined: 22/09/08
standard prop
Just the standard prop it came with when i bought the motor pretty sure its 9 7/8 diameter 11 1/4 pitch. Next step if i'm going to spend more money is to try prop options to squeeze out a bit more top end wether or not its worth it for such a little machine is another question.. Yea I was really happy with the price, doing the motor myself probably saved about an hour of labour i guess and the weld job is a pretty uncomplicated one.
We hired a 420 up in exmouth with a honda 25hp and that was the boat that made me decide to get a tinny as a second boat. from memory it planed pretty easily and didn't bog down, have no idea of top speed though at a guess i'd say it would've been around 18-22 knots.
Dave J.
glastronomic
Posts: 892
Date Joined: 16/02/11
25 HP should get it up on the
25 HP should get it up on the plane with one chap in it.
You might want to add a tiller extention to the outboard tiller to get you further up front in the boat and even the fuel tank further forward so the front does not climbs skywards.
I had the same problem initially in my tinny with a 18HP 2 stroke, after the weight distribution it flyes and gets up to the plane dead easily.
Added a bit of angle and 2 cable ties to the forward/neutral/reverse to give full control from where I sit.
Not pretty, but simple and can be made prettier if you put the effort in.
Good luck, hope it helps you get a lot of pleasure out of your purchase!
Fishwrecked tinny club !
http://youtu.be/C48_834pkpE
Here is a short video of the 3.7 mtr doggett and the 18HP tohatsu a few days ago into the breeze with the tiller mods.
Swan River Fisher
Posts: 877
Date Joined: 03/05/11
how fast were you going in
how fast were you going in that vid? looks like a good set up!
I rekon get a 30-40hp 2 stroke for extra grunt. You could probably do a swap with someone depending on the years of the motors
tigerfish
Posts: 88
Date Joined: 20/08/10
Thanks glastro. Tiller
Thanks glastro. Tiller extension is definitely on the list. I see that you have one of those telescoping ones (with some modifications). I was looking at them in the shop today and wanted to get one but was worried about how well it would fit the twist grip on my outboard as it is slightly tapered. Is the grip on your outboard tapered, and if so, do you find that the clamp on the extension is secure? Had visions of it coming off while at full throttle in rough seas
.
Like the idea of gear selector extension, will have to work up something similar...
glastronomic
Posts: 892
Date Joined: 16/02/11
Yes works well and can be
Yes works well and can be taped on for extra security.
Got mine from Dinghy World @ less than $35 from what I remember, who happen top be a Honda dealer so you could try it there for fitment.
here other angle to give better idea, plus the killswitch cord is also all the way forward where I steer/sit.
It is not fancy but works well, easy to dismantle and tinny is for catching fish and bait.
SRF; do not know but estimate somewhere in the vicinity of 35knots @WOT will check with portable GPS one day!
Brucesta
Posts: 1721
Date Joined: 29/05/09
i owned a 420 dory wide body
i owned a 420 dory wide body with a 25 merc 2 stroke and it certiannly struggled with 3 on board, i can see the 4 stroke being a little worse, move the weight forward if you can and get off the back of the boat, made a big difference for getting out of the hole. top speed was never flash but who wants 35knots from a tinny, that'd be scary.
Las Vegas - Rolling the dice and trying your luck. 1M+ Barra summer target. 100kg Black Marlin winter target
thesupervisor
Posts: 1136
Date Joined: 10/06/09
i would recomend the
i would recomend the disturbuting the weight a bit better as for having 3 people on bord it can be done but you would need a diffrent prop for extra weight conditions to get it on the plane but it will comprimise top speed go have a chat to just props in ossi park he will give you some free advice
getting the bottom line final answer from a bunch of blokes that use false names and put smiley faces at the end of paragraphs is not the best place in the world to get the information you seek.