I wouldn't even try, sorry! The 505 really is too small for such a big horse, and I wouldn't want to risk them panicking and hurting themselves, or having a bad experience and becoming anxious about loading.
From what I believe it depends on the weight of the 17.2hh and the pony to ensure they don't exceed the MAM of the Ifor and of course taking into account the vehicle's towing capacity.
Friend of mine used to carry a 17.1hh and 16.2hh in a 505- they looked pretty cramped and would have been over weight.
The 505's have a MAM of around 2350kg and weigh approx 1025kg unladen, leaving only 1325kg of horse to load.....from what I understand.
I have done this (a long time ago!) but the horse in question was extremely compact and a superb traveller. Even so, her tail hung over the back ramp and she was pretty firmly wedged in place. I might consider it for dire emergencies but wouldn't do it as a matter of course. I think a bigger issue may be the weight/stability of having such a big horse in the trailer and I guess you'd need to check the capacities.
I wouldn't either. My horse is 16.2 in 6'6" rugs and dosnt really fit with the partition in and was becoming increasingly difficult to travel. He now travels perfectly without the partition where he stands diagonally.
Just because you can squeeze them in doesn't mean they are comfortable or safe. Best to take note of the spec, guidelines. I'd also agree with ljohnsonsj's comment.
I wouldnt even try it! There is a reason 505s are for up to 16.2hhs. Head room, weight, width and length. They are not designed for the bigger horse. I just sold my 15.2hh and bought a 16.1hh and am now considering whether to sell my 505 and purchase a 510 to carry her. Am going to try getting a full breast bar and travel without a partition first.
Mines 17.2 ish and he has been in a 505 (only to demonstrate he loaded) he fits in the 510 but only leaves a bit of breathing space. I wouldn't travel him in a 505 as he wouldn't be able to balance let alone move a bit.
My 16.1h fits fine in a 505, he's a 6'6 rug too. He has plenty of space though even when traveling with another pony, although she is only 14.1h! I had. 16.3h mare that used to fit just about, only had her a few months though on loan so didn't see a point in changing boxes, only time she used it was one 10 minute trip a week about 10/12 times. It'd be a fair squeeze for a 17.2h though imo! Nearly easier to get the 510 and save yourself the hassle
We've owned our trailer for 6 or 7 years, in that time my daughter has progressed from an 11.2 (we had to put lower fixings for the bars) via a 13.2 and 14.1. Just over a year ago we bought a dainty 16.1 (according to passport) she's very slim, takes 6'3" rugs but she has grown a LOT taller, not measured but folk say she's easily 16.2 or 3. Obviously she's been travelling lots and is quite happy in our trailer, but I do feel it is a touch small for her now, not in length, certainly not in weight - she only weighs 490kg at her heaviest, but in height I think it's getting too low even though she knows not to snatch her head up when she is on full alert as we arrive at a venue I do always think it's not high enough. So we're aiming to get a lorry this year. We rarely travel her with anything else other than the (also slim) 13.2, and we remove him backwards from the left before swinging his partition right over to the left to give her the room to turn out and down the ramp safely without catching her hip on the exit (when it was just the 2 ponies, you can unload either one first as they both have room)
So I'd say you're pushing it at 17.2 unless a total skinny lightweight who doesn't do the heads up on arrival! Wouldn't do it personally.
I definitely wouldn't do it. There is a reason, as someone said, that they aren't designed for horses that size, and it's completely inappropriate IMO to even try.
If your buying a new horse trailer and haven't got a 505 already why are you even asking ?
Off course you buy the one whose dimensions fit your horse your be foolish to seriously consider different!
Quite taken a back at how many will risk the safety of their horses. 505 are listed as suitable for 16.2 and under if you are not worried about your horse you should be worried about invalidating your insurance for yourselves and other road users as the trailer is not being used "fit for purpose".
Ifor Williams 505 are listed as suitable to carry 2 horses up to 16.2 and under. When talking to the Ifor Williams dealer the only single trailer is the 403 which I agree is not suitable for larger than a 16.2. According to the dealer the main consideration is weight limit. With the 505 having a maximum weight capacity of 1435kg there is not a problem with my 650kg horse. Safety is paramount and only wanting to travel 1 horse in a sensible recommended manner (cross tied with breach and breast bars) is perfectly safe and acceptable. The insurance company has only stipulated that 1.the weight carried is within the limit for the trailer, which with 785kg to spare is no problem. 2. The vehicle is serviced regularly. 3. Is secure and locked.
Just asking as I was offered a lift to something with a chum who has a 505. Id never consider buying one. That would be silly. I thought he wouldn't fit but thought it worth an ask in case loads of you said you do it all the time without an issue.